What's the best combination of skill points?

As mentioned before, whatever skills you want is a great combination. If it was
a useless skill, it wouldn't be in the game. There are, however, some useful
tips that would behoove you to take advantage of. But again, there is virtually
no combination of skills that wouldn't work.
1) Keep it to two trees. A tree is another word for mastery. If you choose
Heuksal tree and Fire tree, keep your outside points to a minimum, as those two
will suck up a good portion of your skill points on their own, even if it
doesn't seem like it at times.
2) Give yourself some offensive power. Any of the level 5 abilities from the
fire/cold/lightning trees are good, because they're a temporary rocket booster
to your damage rating. It's good to get you out of a bind. If you're going
sword and force, you'll be spending more time fighting the enemies if you don't
have anything boosting your attack.

Again, there's nothing wrong with going three elements or going straight
sword/heal, as you can complete all the quests in the game just fine. The idea
is to have fun building your own character rather than begging for advice from
someone you've never met before who may intentionally lead you astray.
http://www.gamefaqs.com/computer/doswin/file/930711/43300