Use your sword to hit the three tentacles and get them out of your way. After that, run to the right edge of the platform and toss a stick of dynamite into a small hole just behind Aquatis by holding the button down. The dynamite will explode in the hole and shake a stalactite loose from the ceiling. The stalactite will fall and damage Aquatis. Immediately run as far to the left as you can to avoid Aquatis' tentacle frenzy then repeat the strategy. Note: There is no need to stand on the small floating platform next to Aquatis.