A fairly big problem with Lionheart is that the combat is, quite bluntly, terrible. I tried playing with a focus on fire magic, but it's frustratingly weak. What's the most powerful build for combat? The story is compelling enough that I want to give this game a fair go, but I also want to be able to get through the combat with reasonable ease.

Went through the game with Fighting One-handed melee weapons and Divine - Fortitude and Divine - Divine Favor. Was a pureblood human with a focus on feats that give skillpoints, weapon mastery and regeneration.
Gave me a pretty easy time, but didn't do a lot of damage, so fights tended to take a while. So in short, this isn't a powerhouse build, but if you're having a hard time, it will get you through.

I use a fighter with one-handed melee and joined the Templars (their quest has a good shield that gives you 10 AC, 10% damage resistance, and 1 point damage threshold), and acquire perks that give me additional damage resistance and AC. When I'm finished with Barcelona, my AC is around 150-160 and I rarely got hit by physical attack. Magic attacks still hurt, though...but not life-threatening as long as you're not facing a group of mages.
Oh, and you can abuse the random treasure chests to get the best weapons and armors if you can stand the boredom of quickloading. :P