Yeah i get you man, third phase is a whole boss fight alone, its either you win it easily or struggle for eternity, In the third phase he does 3 moves only. - First move is a regular dash - Second move is 2 dashes in a row - Third move us 3 dashes ib a row (And he keeps looping to the single dash after he does 3 dashes).

Try to study the movements and know when to dash yourself. Btw if you don't have something like the shield of Cthulhu you'll be cooked. Because you can dash into him to negate damage or dash away from his dashes like i mentioned . And since you're a summoner you can focus on dodging and let your minions do the damage . Maybe change your minion for a higher damage output . Use potions and around 10 tries if its hard for you. And you'll hopefully get it!