There's a few things you can do to prepare for the boss:
- Get complete armor sets. If you have Necro Armor and the Molten Helmet, you can't really take advantage of the full armor set bonus. Find what "role" you play best as (either Ranged, Magic, Melee), and get the appropriate armor set for it. Since you seem to play as a ranged character, considering getting the full set of Necro Armor to give a higher chance to not consume ammo, especially for an expensive ammo consumer like your Star Cannon
- Prepare a skybridge in Hell. Makes the fight much easier to manage.
- Stock up on potions.