Malgus always has one 1vs1 phase with each player (the one where he chokes the other party members). If you want ANY composition to work, just get high enough dps to kill him before he finishes the 4th 1vs1 phase (he won't become immune). Problem solved.

He is totally right... Malgus will have one solo phase with each party member.. kill him before he finishes the 4th one and your problem is solved as well. You won't have to kick him off the ledge.