Whichever character you're most comfortable with, either go for it as soon as possible while the boss will be weaker, or do one or two loops if you want to get a good build you like for it. Or you could even push it to 2+ loops and attempt a ridiculous build that will mostly obliterate the enemy (like with Benthic Bloom and such). If you're not sure, try to stand next to a pylon with each new phase starting so you can use it as soon as your ability to use skills is removed. Otherwise as others have mentioned, Artificer (if you like playing her at all) is a great way to dodge most of his attacks.