I still don't agree that multiplayer should be easier, though -- I'm certainly open to the discussion, but I think that difficulty should remain constant regardless of number of players. That's my core premise and is unlikely to change. New Super Mario Bros Wii got actively harder with more players -- I didn't like that. Most strategy games become a cakewalk if you have too many players on your side. I don't like that. However many players I have with me at the time, I should be able to get a consistent, clear difficulty level. If I'm having to lower or raise difficulty because of playing with different numbers of players, something is very wrong.
And so that's where the current math is doing things right, I think. Enemies almost all have piercing shots, did you notice? So that means that one player can't block a shot to prevent another player from taking the damage. You both have to dodge, but the enemies don't need twice as many shots to accomplish this.
Then you have your damage against the enemy: presuming that all player damage output is able to be normalized to an average (ignoring specific character build bonuses, is what I mean), that means that two players will kill enemies twice as fast. Three players 3x as fast, etc. If you increase enemy health the corresponding amount when there are multiple players right there in the vicinity, then the same amount of damage needs to be dealt to enemies as if each player was playing solo. In other words, there's no disadvantage at all, numerically speaking, and it should balance out to be the same difficulty as solo rather than a lessened or increased difficulty -- which is my premise, recall.
On the other hand, there is one thing already that breaks down the difficulty for enemies in multiplayer: you DO get an advantage, and a big one, already. Namely: enemies don't get extra shots. So if you and your friends all stand at different firing angles from the enemy, the enemy can't fire at all of you at once. Meaning all that piercing and such doesn't help the enemy at all. If you and your friends stay really close together then it's the same as solo, but if you flank the enemy then you suddenly have this crazy advantage already.
Which would be the point of "calling a friend to help," as it were. In short, bugs aside, I think it already does what you want better than it does what I want. Players really can probably abuse the ability to flank the enemies in large numbers in particular, and that's more the problem I'm worried about rather than it being not any easier. So I actually do agree with you that, long-term, there is probably a problem; I just think that it's the opposite of the problem you were thinking.
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