In phase 1 when you’re on the upper platform if you have summons that you can summon on your cursor, like the Acolyte with Summon Wraith with one of the passives. Then you can use that where the boss is, and it’ll spawn on an invisible platform (that you presumably use to place the boss on during the boss fight) which causes the boss to attack to the left.
Also, after the boss fight is over, if you do that, the boss disappears.