Bombs are definitely the easiest way, dropping one when he's close and then running away - at least for the first form. The second form can be hit with any of your weapons, though the pole is probably the easiest. If you're stuck on him, move on; you don't need to fight him until you've beaten the dark area of the cave.
What Matthew said. Use your Stopwatch during battles to view hints like this screenshot.
Also, get well-prepared. Find more Heart Boxes (4 hearts is tough), get a booster injection, buy that Potion, win prizes at the Shack, find more weapons... these little things will help you.
