Could be a repost but never saw a post personally about how its done this way. Just a few let him die at 99% etc
Send magg in to detonate the bomb, and have everyone run out of the room asap, not picking up any aggro. Let him die, and do not rez him. To the right of the closed gate, jump up the rock formation using the torch to jump OVER the gated wall. Do the final boss like normal, grab chest, wait for final reward, then talk to magg. He will start up the 3rd phase while on the inside of the room, in no danger of dieing so afk for 4 mins, get chest from the previous room, and get the boss chest again.