When you actually start to fight shartuul he spawns eye stalks that mindflay you and slow you down. If you stand in the middle of the teleporter with the assassin they cannot hit you.
You can hit shartuul still just fine yet you won't get mindflayed. Theres a small chance you will when he does his magnetic pull but you can easily go into shadow form and shadow nova.
I know in the pictures there aren't any eye stalks up but that's because the event bugged on me and i just wanted to get this posted.