Most effective class in Dragon Age Origins

I've been playing Dragon Age Origins for a long time now, and it seems that every encounter one of my team dies. Am I doing something wrong?

What is the best possible class to play in order to survive the campaign? I could stick it to 'easy' but it seems defeatist.

Any suggestions on which spells or weapon setups and such?

There is no "best" class, for parties any class will do, it's all about tactics. If you're going to solo the game, then a Dwarf Rogue is the race/class to choose, those things are tanks if speced right.

Focus fighting, with parties you'll want to focus specific enemies, not always the biggest baddest one, sometimes it's best to kill off that healer or mage, other times the archers are a pain. Sometimes the big bad guy is the best to always target. Spam potions, they're easy and cheap enough to make, and really can save your life. (Both energy and health potions).

It is certainly possible to beat Dragon Age with any class, but I'd say that mages are the most powerful. The main reason is that they have just superior crowd control abilities, you can freeze the enemies, paralyze them, put them to sleep, put them in a force field and so on. And you can deal some serious area of effect damage as well.

I'm a big fan of a two mage party, but you have to actively manage your mages, setting up the AI for that is not really easy.

I have to say that I played mostly when the game was new, I haven't played much after the patches came out. The advantages of the mage have been reduced somewhat, but they are still very powerful.

Well considering I have seen it being solo'd on Nightmare by both mages and rogues, I think any class combination should be fine, don't you?

As a hint, try not to attack groups of mobs directly, as the game itself says in a hint, "nobody will remember your glorious, but failed frontal assault". Use line of sight, and large area aoe like Blizzard to soften mobs up before they even get to you.