Disclaimer: I just got the book this weekend and haven't played it yet, though I've read through it once.
Anyway, it seems to me like another approach to the simultaneous attack (or any teamwork focused on several heroes fighting a single big bad) would be to use Complications instead of Assets. One limitation with using assets for this specific situation is that only one hero can benefit from it. When there is a single villain and multiple heroes, using complications means every hero can benefit from the bonus.
The first hero rolls and places a complication on the target, say with a d6 Effect die. Then the next one can try to improve the complication, and he gets to include the d6 from the first one in his pool, making it more likely. If he gets a d8 or higher effect, the complication is increased to the new value; if he gets a smaller effect, he still steps it up to a d8 anyway. Then the next one helps out, then the next one. Soon you'll have a d12 complication on the bad guy, which you will ALL be able to add when trying to take him down, and possibly when resisting his attacks. Some players will find this a more satisfying approach than building up an asset that only one of them can use, since they are not only contributing to the teamwork, but also getting a benefit themselves.
Of course, this means you are rolling against the villain's dice pool instead of the doom pool, which might make it harder to succeed early on but (possibly) a better choice later in the Act once the DP has grown some and might be more than the villain's own pool. And you always have the option of not causing stress in your final attacks, and just go right for victory by stepping the complication past a d12, and coming up with a narrative that fits how you render them helpless through your group onslaught.
One nice thing about this approach is that you can get creative with the exact nature of the complication. It could be something simple / generic like "Overwhelmed by Numbers" or something, but that is a little flat and the villain may well have many traits that he can justify using in his pool to resist it. But make it something more specific to the villain, and you might be able to limit their reaction pool too. For example, maybe the complication "Give her no time to concentrate!" would be an appropriate group tactic when fighting a psychic, and it might be hard for her to justify her using psychic powers to resist, and once applied you would get to benefit from it on your own reactions against her attacks. Similarly, something like "Keep him off balance!" for a brick (durability/stamina probably won't help him to resist, and you can use the complication dice on both your own offense and defense), or "Hit him from all sides!" for simulating an actual simultaneous attack, which might also make it harder to justify adding Super Reflexes in the reaction pool so it might be a good one against speedsters, etc.
Naturally, it will be up to each group to find their own balance in what complications would or would not allow the target to use any given trait for their reaction pool. But wherever your group decides to draw that line, you'll still have room to play and get creative and find ideas that work for the players and also for the specific character concepts involved. And you if end up just trying to step the complication up past d12, let the players decide how to narrate exactly how they remove his ability to act.
For example, there's the classic fight between Spidey and Juggernaut, where Spidey just cannot manage to hurt Jugs. Even his webs aren't working as complications, because Juggernaut can break out of them using his Godlike Strength. But he eventually tricks Juggernaut into the construction site of a skyscraper and traps him in the concrete foundation that had been just poured. There are probably a number of ways to represent that in MHRP, but one might be that Spidey finally realized he couldn't outfight Juggernaut, so he went for a complication of "You're Right Where I Want You" or some such. Juggernaut probably can't justify using his Godlike Strength or Durability in resisting that, so Spidey has an easier time applying it, and each time he succeeded on stepping up the complication, he narrates how his "attacks" and defenses and overall movement are leading the villain towards a nearby construction site. Then when he finally steps it up past a d12, he's succeeded at making Juggernaut helpless in some fashion, and he narrates it as tricking Juggernaut into the concrete.
Anyway, that was my take on a good approach for a team taking on a "boss" villain. If I got any of that wrong, please let me know.
