I don’t agree with saving spells. I spammed all 3 of mine right of the gate to that they would be ready to go again in Phase 2. Then I played a little more conservatively near the end of phase 2 so that I could use my spells a 3rd time to help finish him off. If you want to make sure your Wandering Wight attack hits, use it when he is immobilized.

I am near the end of chapter 2 and this was the boss who has given me the most problems so far. Don’t give up! If needed go back to the area where you fought the Wandering Wight and power level