So it seems like there are quite a few people having trouble with the Vapor two-bird in 4.4 after 5-starring the mission. It's a popular fight because of PKB's very well written guide to farming CPs. I thought I'd write up a post with some tips for the fight, since the page has well over 250 comments, which makes finding useful information for the fight difficult. There's also a lot of less useful information such as "just use Ghost Rider she is easy", which you won't find here. This last statement, while true (and is also how I usually do the fight), isn't very helpful because not everyone has Ghost Rider and provides zero insight into the actual fight.
Easiest Possible Way for Those Who Don't Like Reading
Just use Force Cage on Vapor. I know not everyone will read this entire post (it's pretty long). That doesn't mean I don't want you to get help! For those of you who won't bother reading the rest of the post, this is all you need to do. It's not the fastest way, but it definitely works. If you still lose when doing this, there's nothing I can do to help you. I've never tried the fight without owning Invisible Woman, so I'm not sure what level she is on her team up. But you should definitely get her, it's a waste of XP not having her since she's required for the fight and she's only 15cp and well worth it - Force Cage works well on many other boss fights too. You should have at least 5cp from 5-starring the mission, which is why the fight is hard anyways.
When caged, she'll just change state 3 times, and then do nothing. The bruiser and Fixer are easy targets since Fixer has no real attacks, and the Enforcer has two attacks that don't really do anything special. After you finish those two off, it will be 3 on 1 against Vapor.
There was a commentor who had suggested caging Fixer - normally I try my best not to point out when someone has different opinion from me, but in this case, that is definitely not the right way to do this fight. Yes, his bombs are annoying, but (1) they're predictable, (2) they take him two turns to use, and (3) they don't hurt very much. Vapor is much more dangerous. If you cage Fixer, I'm reasonably sure his first move will be Upgrade (it's been a while, but I'm pretty sure this is what happens). This makes your fight really hard since Vapor's poison attacks will now cause Nanoplague.
Understanding What Happens in the Fight and Why it is Hard
Here, I'll talk a bit about each of the enemies in the fight and their behaviors. Hopefully, this will provide some insight into why the fight is actually hard. I'll be the first to admit this fight can be a bit difficult, but I've seen more than one commenter with the wrong reasons for why they're having trouble. Ideally, if you know why it's hard, you can even figure out your own solutions. I realize not everybody is interested in reading through this part, but I've included it here as an example of how to critically think through a fight (something commonly advocated for by PKB, and justly so - some of the things I see on his wall...). Hopefully you'll be able to figure out some of the other hard fights on your own - e.g. Mephisto from SO4 (admittedly kind of late for this one!), The Hood Epic in 8.5, Excelsior Abomination 2-bird w/ Exoskeletal Battletank in 9.4, Dormammu in 9.6. If you're genuinely not interested, just skip ahead to the next section.
He has three attacks. An area attack (Sledgehammer Roundhouse) that doesn't hit very hard, a single target Stun (Induce Trauma), and Crushing Blow, which causes Weakened. The roundhouse isn't a big deal, the stun and the weakened can be annoying. Of the three, definitely causes the least amount of troubles. The stun can be a problem if it comes at the wrong time. The area attack can be a big deal if he gets enraged from a Scrapper, so be careful when using Ghost Rider's Burn Out or Luke Cage's Ground Pound. Other than that, he's not that big of a problem. If the fight were 3 Enforcers, the fight would be easy. So he's definitely not the problem. Honestly, I usually just leave him alone, he ends up dying from Vapor / Fixer.
Fixer throws grenades and upgrades. It's important to know when the grenades go off because then you can prepare for them. He will always throw the grenades on one turn, and then on his next turn he will use Blast Shield. The mines go off at the start of the round after he uses Blast Shield - NOT before his next turn. This means if Fixer is the last person to go, he will use Blast Shield and then the mines will go off. The easiest way to keep track of this is to remember who took the first turn when the fight started. If your Agent goes first, then the bombs will go off before your Agent goes and after Fixer uses Blast Shield. I'll repeat this again, it's important to keep track of when the greandes will go off. If you don't, you can lose the fight quite suddenly (see the part on Vapor).
As for upgrading, it changes Vapor's attacks, but I don't think it makes him stronger and I haven't noticed any changes in the Enforcer's behavior. I'm pretty sure he does this ONLY after Vapor has taken a certain amount of damage (not sure how much, but I haven't seen him do it with Vapor at full health), and he only does it on his next turn when he's not in the middle of a grenade cycle e.g. he'll never use it instead of Blast Shield on the turn after throwing grenades.
There's a lot to say about Vapor. She has two forms, poisonous and explosive. In poisonous form, her attacks cause poison and will also counter melee attacks. In explosive form, her attacks hit harder and she will counter energy attacks. But none of that is what actually makes her hard. There are bosses that cause status effects, there are bosses that counter attacks (Vector - he counters everything, not just ranged or melee), there are bosses that hit hard. Obviously there is something special about Vapor that makes her hard.
There is one thing that Vapor does that's hard to notice until after you see it a few times. When Vapor's health gets low (less than 40% or so), she will always change state and all of her attacks become area attacks, including counter attacks. This means if she is in explosive state, and then you attack her and her life gets low, then on her next turn, she will change to poisonous form and she will do two area poisonous attacks. This isn't that big of a deal since poison doesn't stack.
The real danger occurs if she is in poisonous form and then changes to explosive form. The two area explosive attacks can be a big problem (easily 2/3 of your health bar). If you're poisoned to begin with, that's another 10% damage when your turn comes up. Remember, the trigger for this behavior is her health - it doesn't matter if it is from your attacks, or from DoT's or from Fixer's grenades (see why it is important to keep track). Once her health gets low, on her next turn she will start doing area attacks.
The only other thing worth mentioning is nanoplague. After Fixer uses upgrade, her poison attacks apply nanoplague instead of poison. You could end up with both!
- Get a Quantum Jumper. In the words of Xobai, in a turn based game, extra turns are a ticket to winsville (he's since updated his guide so that it no longer says this, but I still remember!). It's definitely not impossible to win without one, but it helps quite a bit. If you're not going to have a QJ, what do you need more CPs for anyways? You want more turns for the heros you bought with CPs, of course!
- Bring a Scrapper. Vapor is what makes the fight hard, not Fixer, not the Enforcer. Vapor. Don't bring a blaster "to one-shot the Enforcer."
- Ghost Rider is the top choice because his damage gets real high real fast, especially with Vapor's two turns and counter attacks, but not everyone has him.
- Sif is a good choice because she is immune to poison, and the weakened from her Flying Sword also helps a lot. The upgrade to bleeding should make the fight go by quickly. Iron Fist is fine, Luke Cage is fine, ANY SCRAPPER IS FINE (even Spider-Woman), just bring one.
- Your scrapper will be your main damage dealer. He'll be a crappy damage dealer if he is weakened. (Don't forget, poisoned also lowers attack!). A gene innoculation may not be such a bad idea.
- A scrapper hero and a scrapper agent may be a bit of overkill, since the Bruiser has an area attack. If he attacks hits one of your scrappers and then you somehow hit him with another scrapper (distress signal, carelessness, area follow-up attack from Quicksilver), his next attack will hurt a lot. This is probably not worth the risk, especially if you have a QJ. Your agent won't be attacking very often anyways.
- Don't bring a Tactician. Vapor is what makes the fight hard. If she attacks your tactician, all her attacks suddenly become 30% stronger. There's no need to make the fight more difficult for yourself. I realize it's tempting to get an extra attack off on Fixer, but he doesn't really do anything dangerous.
- Use a Detoxifier. You don't want one of your characters to die from poison after surviving two of her area explosions. It also makes curing easier, and you won't have to be as afraid of her counter attacks (most scrappers are melee). It lasts three rounds, so it's not even a bad idea to use it in advance. That's 10% of your character's HP that you don't have to heal back later. Poison also lowers attack, and you really want your Scrapper to be at peak damage output.
- Use a Tactical Strike on your scrapper. Combined with the scrapper double attack bonus, this means 4 attacks per round on Vapor (the main enemy you should be focusing to kill quickly). 6 per round if you have QJ.
- Try to limit the number of turns she takes when she is below 40%. This is involved (and will require paying attention, god forbid), but is the key to an easy fight. Some things to remember:
- The best way to limit her turns below 40% is to kill her before she gets any turns. This is partly why Ghost Rider works so well - with a Burn Out + Penance Stare you can go from 60% to zero without her ever getting a turn. A Tactical Strike will also help you accomplish this. Four attacks from any Scrapper (okay maybe not Quicksilver) should easily be 40% of her life. Combined with an attack from your agent and a distress signal should do the trick.
- Keep track of when Fixer's grenades will go off. You may keep her at 50% life, but then she drops below 40% from the grenades. If you're poisoned when the grenades go off and causes this type of a situation, you could end up with a very low chance of winning, especially if you don't have any turns between Vapor and when the grenades go off.
- Don't forget to factor in DoT damage. If Burning or Dark Void (Magik) bring her life below 40%, the effect is the same.
- A Force Field from Invisible woman before these turns should be enough to negate the first area attack (even the explosive one). Healing before it comes should also help.
- If you have a Quantum Jumper, make sure you are bringing her life below 40% on a round that your QJ is up. This should help limit the number of her turns to just 2. This may mean attacking fixer for a turn while you wait a round.
- When her life is low, her counter attacks will also be area attacks. So be careful when attacking her.
- Be prepared to heal. This may mean bringing a Curative Reach, or having a Team Health Pack ready to use. Unless you have Ghost Rider, it's very likely you will need to heal at some point. You might also consider healing pre-emptively.
- The Future Foundation outfit for Invisible Woman definitely makes the fight easier (it's quite a good buff for PVE in my experience). I think the QJ helps more, but this definitely helps. At only 30 CPs for the Infiltrator version, FFIW is one of the best values at 45CP (you'd be surprised at how many boss fights become easy with Force Cage). The Blaster FF uniform is better for this fight for the guaranteed crits on the Enforcer. Neither uniform is necessary, though.
Hopefully there's enough here to help all players defeat her. If you have a tip that I've omitted, feel free to leave one in the comments and I'll update the post if it's a real tip - e.g. I won't suggest bringing Human Torch (I saw like three comments with this suggestion!)