If you're having trouble with Dragon Age Origins, then there are 2 key classes that are incredible in damage dealing and damage immunity respectively, depending on your play style.
Dual-Wielding Warrior/Champion/Templar:The goal of this class is to do as much damage as possible. Get momentum as fast as you can early on, then never have it off, because it's the only reason that this class is better than 2 handed. The incredible amount of damage allows this class to single-handedly destroy most high priority targets such as mages, which is why the Templar class is necessary. Champion is grabbed to reduce damage taken by your party in large group fights, buff them, and allows you to have a bit of crowd control on the front lines.
Mage/Arcane Warrior/Blood Mage:With this class, you buff until your eyes glow(literally)and take enough hits to kill a Brumak and being none the worse for it. In the beginning, chug as many points into magic and constitution as possible(about a 3/2 ratio in favor of magic)because your magic stat replaces your strength stat when you get the first perk in the Arcane Warrior tree. Once you get to level 7 and get Arcane Warrior, start investing as many points into the Arcane Warrior tree as possible, and grab Arcane Shield, Rock armor, and spell shield among other things. At this point the game becomes a bit drab, because your buffs will have made your mana pool practically nonexistent, but with some good armor, a sword and shield and decent support, you will be able to take any hit from any enemy save perhaps a high dragon. Once you get blood magic, then the fun begins, because with blood magic on, your health is your mana pool free of buff restraints. And while taking your own health seems like a bad idea, with your now formidable constitution stat and damage resilience, you can throw around spells with impunity and have little worry about dying. It's best to be careful, however, and it's best to have a spirit healer around to make sure you don't need to keep drinking health potions to keep your health up.