@Glomoro agree with not collapsing the roof, it could make them trapped forever. But they could collapse the ground instead, then fly to the other side of the room. Even if not all the ground, a huge portion of the enemies would get fucked if a big hole is made in the middle of the room. There are no pillars holding the top floor for one.
Also while I understand that Well could teleport out but not back again (because he doesnt knows where they are exactly), he probably could teleport out of the dungeon and use the trap to get to the bottom over and over, as it uses the target's own magic (absorbed) to trigger. Cant him teleport more than one person? Even in one by one trips it should be fine?
And about their fighting plan... 'whos gonna be the vanguards... lemme see, I guess the three fucking melee fighters?'... no shit, Sherlock?