Also, make sure not to complete the weakening gate quest unless you are *sure* you can take down the last boss. If you turn in the quest inside, you won't be able to get back in as there is no way to get the runes to weaken the gate again.
Well, you can't tell that you can take the boss before you go in... And once in, it is too late.
But, you don't have to die if you know what is awaiting you. The way that fight works is that you have allies - the NPCs that you were talking with as you progressed through questline. And one of them has a way to win the fight whether you participate in it and put yourself in danger, or if you stand by the side and wait for the that ally to say "OK, time to win".