Iku isn't as overpowered as I originally thought, though the revelation of this comes from a better understanding of the midscreen game.
Iku is scary at midscreen, as we all know. She has crazy range normals and a very fast 6c that punishes the startup of so many attacks that it can even beat graze attacks with laggy startup (re: China). She seems uncounterable at first, but that is just a poor understanding of how the midscreen game works.
Remember that fundamentally, the midscreen is about getting to some position where your opponent can't danmaku punish, then throwing danmaku. This is the gameplan of EVERY CHARACTER IN THE GAME. Therefore, Iku's 6c -- a danmaku punish -- is just something you have to account for with positioning. If she can 6c you, don't throw danmaku. Actually, don't do anything. If she throws it, just graze up or down and she's just thrown away a bar of spirit for nothing. As Iku, you want to try and get into that punishing position as much as you can, but if your opponent does nothing but safe danmaku, Iku loses the midscreen fight if she only throws 6c.
However, Iku also has the same plan; get to where the enemy can't danmaku punish and throw either 5b or 5c, depending on enemy screen position. If 5c would be valuable, then throw it, otherwise 5b. Iku can also use her alt 214 skillcards as extra tools in these situations.
Iku's 214 is her most important option in matchups, so running the right one is important. If you run DGLF against Reimu, she'll laugh. Even ranking the default 214 is better in that matchup (not much better though). This means Iku should have several different decks to account for matchups. The main deck choices are DGLF or Dragon's Eye, so at least 2 decks.
You'll want to run DGLF in any situation where your danmaku clearly loses AND where winning the danmaku fight wins the match. Alice, Yuyuko, Yukari, Patchouli, Komachi, Tenshi, Sanae, China, Suwako and Utsuho are all characters you'll want to run DGLF against. It is also the best choice against Sakuya, but DGLF won't win the match for you. Against everyone else except Remilia and Aya, you'll want to run DE. Against Remi, DGLF is a little better of the two cards, but you may want to rank Veils Like Water (to level 1) instead to have more autoguard frames. DGLF works okay though and helps out in some scenarios in that matchup. Against Aya, neither skillcard is very good; I don't have a good handle on that matchup yet. DE is probably bad in that matchup though.
Anyway, Iku has a unique advantage in most matchups (other than Alice) because her j2a has huge range and beats most things other than anti-air danmaku. Some characters with no real anti-air danmaku (eg. Utsuho) have huge problems with this simple approach. In general the safest option against an airborne Iku fishing for a jump-in is to jump back or use anti-air danmaku. For most characters this is a B bullet, such as 2b or 6b. For a few characters it's a special, such as Yuyuko 421c or Komachi 214. If the enemy is using anti-air danmaku on a regular basis, just setup danmaku before you get into range or air backdash and then use setup danmaku such as DGLF or 5b. Assuming the opponent also BDC/HJC7 and threw bullets, you're more or less even, obviously disadvantaged vs. Yuyuko.
If the enemy just jumps back when you approach, it can be tempting to 6c but that only works if your opponent isn't Utsuho (5c is too dangerous) or is being predictable with setup danmaku. If the opponent does not understand that being parallel with Iku is a bad time to danmaku, YES THROW 6C UNTIL THEY GET IT.
If both players play smartly this will result in a whole lot of nothing happening. Bullets, grazed bullets, landing into more bullets and possibly some melee fishing, especially for Iku. DGLF is very handy in these situations because it beats many bullet options. DE is handy in a few situations where the opponent has no good answer to it; the Reimu and Marisa matchups are good examples of this. Only throw DE if your opponent can't easily get in and hit you; ideally this will be a situation where both girls are in the air, or Iku is in the air and the opponent is on the ground.
If both girls are on the ground, Iku is at advantage in most matchups. Against Alice or Yukari things change a bit, but against everyone else, Iku's big melee hitboxes (3a/6a) beat most melee rush approaches. Use the correct move for the matchup; don't use 3a against girls with hoverdashes or hops and don't use 6a against girls with low slides. Patchouli can also be a problem with her 66b. Iku shouldn't actually dash in much as her 66a is 15f, which is kinda slow relative to 12-13f dash normals. If you land first and are sure the opponent will try to dash melee, 66a is alright. 66c is also decent if you're expecting the enemy to stick out some kind of low attack. Be careful, as 66c does not hop over the tip of Yukari 3a/66c. 66c is pretty good as a meaty as it has generous active frames and if it hits meaty, Iku is at advantage. You can also use it from very far away (roughly a normal screen width) for a meaty hit, but only if your opponent is expecting a 6c or something and is waiting to graze.
Against Iku, dash-in melee is very dangerous unless you're Alice, Yukari, or Patchouli (in which case, rape her face). Again, it's better to just (high)jump and setup danmaku.
Iku doesn't rush down. She can "get aggressive," but she can't lock the opponent down without leaving a million gaps.
Like every other character, Iku should get on the opponent whenever a bullet is blocked. The standard safe mixup is 2a into 3a/6a into a bullet. I feel that the best option overall is 5b. After 5b, hjc8 j2a at minimum height puts the fear of God into the opponent. You can also use 6c, which is airtight, and do the same thing. You could drill them but FUCK THAT. Minimum height j2a leads into actual damage (I think 2c, d3 j5a j6a combos, but it's somewhat spacing-dependent), where a drill does not. A drill also doesn't look like a rejump pressure attempt. A drill also isn't safe on block if it's bordered.
On airblock, it's the same old j5a j6a 6c d9 shit. Don't do j8a unless you like giving up pressure. It's fine if it hits, obviously.
Don't throw swimming dragon fish SC unless your opponent is knocked down or blocking 5c. The recovery is really long and your opponent can trade 2k off some ghetto 2-move combo for 700 from one hit of your SC.
--Dragon's Eye is mandatory in this matchup. DGLF just postpones the inevitable rushdown.
--Runaway like crazy unless Reimu makes a mistake and 5b whenever safe. Her 5c will beat it, but at least it forces a reaction from her. Keep doing this until you draw DE. Reimu is very strong in this matchup without it.
--If DE is on the screen, Reimu can't do anything unless she draws Youkai Buster. Even then, she basically has to spam it randomly in the hopes that she catches you in DE startup.
--Pressure heavily if you have DE onscreen. Almost a free win once you draw it.
--If you don't have DE, Reimu's barrier makes this matchup very hard.
--DE is the best choice for this matchup, though it doesn't win the match for free.
--Iku has strong advantage midscreen. Just play your normal game.
--This matchup is 100% retarded. Extremely Alice-favored without DGLF. Probably still Alice-favored, even with it.
--Don't try to fight Alice on the ground. Jump back or die. Even that's punishable if she uses HJ9 6a.
--Iku close air seems strong against Alice, but Alice can jump back 6a. Fucking 6a.
--If Alice ever whiffs 6a, punish hard with 6c or something and get in her face. LOL@Iku getting in someone's face
--You want to 6c a bit more liberally in this matchup in order to deal with dolls. Once you have DGLF, use that instead. Whore the fuck out of that shit because it's pretty much the only way you can win.
--I don't really know what to do in this matchup (from either side). Sorry.
--If Sakuya plays defensively, it is basically impossible for Iku to make anything happen. C knives beat DE (cleanly) and any slow bullet is extremely dangerous in this matchup.
--5b and DGLF like your life depended on it.
--At least you're at advantage on the ground, except... Sakuya can just Magic Star Sword to beat 6c and Iku has no graze attack. FFFFFFFFFFFF
--On the plus side, if Iku gets to a close air position, Sakuya can't do much (2b is kinda slow). Fish for that spot CONSTANTLY. Above Sakuya = mostly safe. Use that positioning to mount an offense, no matter how you get above Sakuya.
--BEWARE OF DP DURING PRESSURE
--If Iku plays very defensively, she is disadvantaged, but her strong melee makes it hard for Sakuya to capitalize.
--Don't run out of airdashes.
--If Sakuya has some alt skill bullet that hits upward (I don't think she does?) that is air-usable, this matchup became fucked. I think running Square Ricochet helps with the burden a bit, but it's ground-only. Definitely worth looking into.
--Nothing special, DE = win?
--Youmu 6c is deceptive. It will beat basically anything you do if you're in range. Be aware! If you block 6c, you can do whatever you want unless she has the Myon alt 22. Probably worth using in this matchup.
--Due to 5c/6c, it is hard to get into close air vs. Youmu. She also has a melee-invincible DP.
--Youmu likes the ground and can slide under 6a. Kinda scary. Also her B bullets are a huge problem if you're on the ground. Take to the sky as much as possible, but beware of 6c and spend time doing nothing.
--Youmu pressure is pretty ghetto. Don't be scared by it.
--Pretty even matchup.
--Remi's normal antiair approach (dash j5a) doesn't work on Iku; be aware of her attempts to do this and melee in advance if she is at the appropriate angle.
--However, Remi 6b is a problem for Iku jumping in. It gets Remi on the defensive, though.
--Remi's alt 214 spears make it very hard to do anything as Iku. 5b safely.
--Block on the ground. A blocked Remi 236 forces Remi to supercancel, a hit gives her a free combo into super. You do the math. If Remi has no super or Hakurouken, punish any blocked 236 grazedash.
--If Remi is close to a wall, she may use her 22 grazedive in the air to hit you. Cancel your bullets safely and be ready to block. If you block it, Remi is in trouble.
--Remi pressure is frustrating. Good luck?
--IMHO Yuyu favored (JP thinks otherwise, why?)
--Safe Yuyu midscreen is hard to do anything to. Her 5b is actually a problem in this matchup (wut)
--Even one safely fired C bullet is an issue. Graze defensively and don't let her get close on your terms. Fish constantly for DGLF or 6c counterhits.
--You are mostly at advantage on the ground, but Yuyu can be ballsy and attempt bullets; your answer is 6c, her answer is DP. Rock-paper-scissors. If I were Yuyuko, I'd 5c (1 bullet) and cancel into C DP to bait the 6c. Not sure if it'd work at most distances, but if it comes out at all, it's a mostly safe DP. 3a doesn't work on Yuyuko dashing (she has a hopping graze attack), so use 6a for any defensive melee on the ground.
--Anything blocked by Iku probably means bullets onscreen for Yuyuko. Graze defensively and try to punish the startup of more bullets. Defensive grazing is your first priority, punishing is second. REMEMBER.
--This fight is troublesome in general without DGLF. 5b until you get it, then use DGLF anytime you'd 5b once you have it.
--Probably Iku-favored with DGLF. Slightly Yukari-favored without.
--This matchup is boring like Iku vs. Alice is boring.
--Yukari B bullets are really powerful until you get DGLF; try to fish for 6c startup punishes as always.
--Yukari rules the ground and can DP on reaction to any of your rushdown. I think the drill beats it, maybe.
--Iku rules the close air position, but Yukari 66a has a weird and potentially dangerous hitbox. Make sure your j2a is aimed a little lower to the ground and not at Yukari's head.
--Oni Spirit Bomb is really strong in this matchup, like a faster DGLF. You know how DGLF works, so pretend Suika has a better version of the move.
--Although I said to run DE above, I'm actually not sure what alt card to run in this matchup. DE is alright, but Suika can deal with it better than most due to her ridiculously silly backdash and OSB/fire oni. DGLF isn't useful really though; possibly the lightning "hold" or antiair grab?
--Iku rules the air, just beware of OSB.
--Even or Reisen-favored. No good alt card choices. Delayed lightning is probably ok. Coins seem good.
--Reisen has few options while Iku is in the air, other than 6b or 6c. These options are kinda good. Just sayin. If you have DGLF, it will give you some gimmicks but not really a reliable thing.
--On the other hand, Reisen d214c pretty much ruins your air game. NVM?
--Don't let Reisen get in; play defensively and 5b or DGLF spam.
--"Everything you can do, I can do better," says Iku to Komachi
--DGLF or DE are both fine. DGLF requires more skill (lol) to use.
--I dunno who is favored here. JP says Iku; I don't think it is. Probably even!
--DE is probably the better card but Tenshi can actually beat it.
--Tenshi 5c beats most of your stuff, so fishing for 6c is counterproductive. DGLF still works, though not very well.
--5c actually controls Tenshi's strongest zone, so it is actually decent for zoning in this matchup (it's actually ok in other matchups but only if the opponent is close to where you throw it)
--Tenshi has an alt DP. Just bait it and backdash. The risk/reward is greatly in your favor there.
--Just play normally, but with less 6c fishing than normal (eg. basically none).
--Horrible for Sanae...until she draws alt Kanako, then ffff-
--DGLF mandatory here.
--You probably want to be aggressive. Sanae is strong when zoning. 6c fishing is better than normal, but Sanae 5c kinda wins.
--If she draws alt Kanako, conserve your airdashes. Border up if you're caught blocking it on the ground.
--Sanae wins the zoning game if she has alt Kanako. You'll have to rush down, but her defenses are weak so it shouldn't be too hard.
--Iku favored, bleh
--Close air is beaten by Cirno 5c and the ground is hard to deal with her ice slide. Win the danmaku fight; it's not hard, even for Iku.
--DE = free win.
--China so bad
--I say DGLF in this match since it beats rings, and China rushdown is dangerous enough you don't want to get caught in DE startup in the air. DE is still good though.
--China 2b is actually kinda scary as antiair.
--Nothing else China does is particularly scary, though be careful of blocked dash attack > special.
--I used to think it was bad, now I think it's "less bad." Still bad.
--Utsuho wants to jump back a lot in this matchup.
--If you're both on the ground, she's at advantage most of the time since she is not afraid of 6c and you are hosed (by 66c) if you jump too early. If you throw 6c on the ground, she will eat you alive. If you don't do anything, she will get a 5c for free. It's a bad state of affairs.
--DGLF in the air is how you win. Otherwise Okuu 5c is too good. Beware of alt 22 if she's on the ground.
--Okuu must backdash if you are in close air position.
--Not as free as some of Suwako's other matchups. Still pretty bad for Suwako.
--Your normal bullet zoning is of mixed value here. It's not as good as in other matchups. DGLF will punish Suwako if she does anything in the air.
--Being on the ground is annoying. Avoid it and jump back a lot. Unless you are very close to Suwako, you are at advantage but she can dash under most of your stuff. Annoying.