Whenever a tentacle has little blue dots floating around it, quickly slash it. Watch the eye occasionally when the blue shield is gone. Get in quickly and give it a good slash. This is the only way to damage him. When the eye starts to shoot out different colored balls, go to the right side of the screen and stand in the depression in the ground. Run circles in it, and most of the balls will miss you. Dodge all of the tentacles as they whip around. Use these tactics and you should survive.