I've played through all of the campaigns using warrior type characters, and I'll second what Petrell said: concentrate on a single skill tree, then if you have a few points left over put them into a second skill with the understanding that it will be pretty much purely for support purposes. One thing to keep in mind is that at higher levels the attribute requirements necessary to advance the skill trees become much more stringent, so if you spread your attribute points too thin trying to boost, say, strength and stamina as well as wisdom and intelligence, you'll find you can't meet the requirements to advance your skills of choice, even if you have the skill points to do so.
If you're going for a warrior type character, then sink your attribute points and skill points into heavy combat arts and keep that maxed out first and foremost (also pick either bladed or blunt, getting both is just a waste of skill points- you'll find plenty of good weapons regardless of what you choose). Shields can also be a bit of a waste, since the best heavy weapons tend to be 2-handed; this changes a bit in the expansions, but I found that wielding two 1-handed weapons worked better than going sword and shield. If you want to dump extra points you have into a magic school then I'd recommend going with either White magic (Life or Boons) or Death Magic (any of the three) and use one of the available auras in a support role (you simply won't have enough skill and attribute points to have strong magic as a warrior, but even low level auras can be useful). Unfortunately I don't think Elemental magic has any auras, so points in there are a complete waste (any spell you'll be able to cast will be weaker than your regular attack).