It uses a form of the prayers and changes colors to show what it is weak to:
Orange means you can melee.
Green means you can range.
Red means you can use fire spells.
Dark brown means you can use earth spells.
Blue means you can use water spells.
White means you can use air spells.
It also attacks with both range and melee attacks, the range attack is weak so always use protect from melee.
Switch between melee, range and magic as the monster switches colors.
Just keep your prayer up and keep an eye on your hit points.