When fighting the fire armor thing when you just got Glacius, to avoid the shooting fire rocks when he opens up, stay directly in the middle. You should be able to avoid almost all of them.

Start with Flipendo until he opens his mouth, then use Glacius until he sort of explodes (this does not mean that he is defeated). One of his arms will come off. Use Glacius to put out the fire, then use Flipendo to destroy it. Repeat until he is defeated.