Forge tips?

Hey,

I'm currently in the underdark and I've worked out how to turn on the Ancient Forge. I have two pieces of mythril to turn into something shiny.

I'm a bard 6, Shadowheart 5, Gale 5, Karlach 6. Three resist fire potions.

Problem I'm having is that I can't push enough damage though the golem before I die to it/mephits.

He's immune to almost everything I cast and resists everything else. I noticed the bludgeoning weakness but can't take advantage.

He takes like 8 damage from an upcast Shatter.

What am I missing?

I think I need to come back later.