Mages are extremely strong with numerous broken subclasses. There are like three good archetypes, with the rest being bad: DW rogue/fighter and wizard. 2H warrior is not good until the expansion, which adds some broken abilities for the archetype that make it good. Shield warrior is just for CC, but is also bad compared to just having another damage class. This is all from memory because I haven't played DA:O in a million years. The game is not hard no matter what difficulty you play on, but if you haven't played it before, it's not going to be clear that some classes are just awful because Bioware didn't have anyone that could do arithmetic. Almost all buffs stack, so if you have two wizards in your party, you can haste your party 2x. Blood mage is probably the most broken subclass, so skip it if you don't want to break the game entirely. There is a bug/unintended thing with the Regeneration spell and blood magic which effectively gives you endless mana.