Don't waste your time with Wall. Just beat the crap out of it as quickly as possible with attacks, jumps, Virus, or Ice-3 if you have it. Even have Rosa shoot the bastard. If you waste time doing things other than damage, the Trap Door is more likely to turn into a monster which will lay even more hurt on you. Don't try to keep all your members alive, as it is not likely to happen. Best case, you should be able to take him out before he kills your second member, depending on what member was killed first, but remember, (in my experience) he will kill no more than 2 members before transforming.

DCI Forever!!