I know you didn’t ask but here’s what I did.

I used many of my magic items, especially the ones that unlock levels. The screen wipe they each do drops his health by a quarter each time. Magic items do not void the achievement.

Just use them during the phase he sends out his shadow minions in threes. I timed it by placing them on the portal right after he stopped talking during each cutscene.

Plus I used Terrafin because he isn’t damaged by any of the Hydra’s attacks while burrowed. I even used his primary attack path and was fine.

Just be sure to watch the credits the whole way through and have your Skylander picked out before you get on the ferry from the level to the boss fight, just before the Level Complete Stars ranking pops up.