Keyword research is both difficult and tricky. The only way to effectively refine your list is to test it in practice (PPC campaign may help you with that).
Your competitor may have already tested the keywords and chosen the best ones that both generated good traffic and converted. I do not suggest relying on his keywords completely. But if you take time analyzing and comparing several of your competitors’ on-page and off-page keyword targeting, you can make your list much better.