The Visitor Event Guide/Discussion.

Randomly, not sure if people figured this out, but the only part about the final boss to worry about is time. If you can kill him in the time limit you can beat him always. He'll block anyone who moves before he actually appears on the screen in my experience, the rest are free to fight until that person dies, so try having one person move up right away to avoid other damages and be a block mule. The rest of the party can kill him after that. And as long as one person who's dead stays in, everyone else can re-enter where he is.

I dunno if his behaviour changes when only 1 person is in the boss map, but I have soloed it on my Paladin without any other people in the room. The blocks don't last long enough to kill me so it's just a matter of hitting him in between blocks. He doesn't have that much HP so they add up quickly.