The Sprite Guardian helps and using fire attacks/converts will help out allot. Be sure to put multiple converts onto Gristletusk when using the Sprite Guardian strategy. Polymorphs and really help and you should defeat Gristletusk's minion before you defeat him.
When Gristletusk is below about 8,000 health, he stops using his powerful, hard-hitting spells and, instead, uses allot of his natural attack and Nature's Wrath. If you cast a shield or charm (this includes dispels, feints, and converts), he will automatically cast Hex onto you. (Gristletusk will not use most of these Hexes.)
Hope I helped and good luck defeating Gristletusk!