How do you kill the twins as a mage in expert mode?

I'm not a Mage fan, so I can't tell you.

I managed to beat all the mechanical bosses

With what class?

Mage/ranged I couldn’t figure out how to kill them without the stormbow

Sky Fracture, Rainbow Rod and Meteor Staff are really strong magic weapons that could do some good damage

and the crystal serpent?

That's one mage weapon I don't know

yep, its obtained through fishing in the hallow (underground or surface idk). It fires a shard that splits into smaller shards

Interesting