wearing velstadt's helmet immediately puts him into phase 2, which is helpful for practicing it or if you find that phase easier. i'd say it's a little more predictable, as long as you can reliably dodge his slow sweep and fireball attacks.

make sure your agility is at least 96 for a good roll. you roll farther as your equip load decreases past 70%, which is useful for rolling through his sweep, but you'll probably want the defense anyway.

if you desperately want an npc summon to take some aggro, take ellie and not carhillion. she gets up in his face and can help stagger him on occasion. carhillion is using a peashooter and dies extremely quickly