The game levels the enemies up with your PC (player character) so if you level up your PC, but none of the other characters, then the rest of your party will die frequently and easily.
As for stat building, here’s a basic premise, as even though there are several stats, only a few are even important enough to bother with. For mages don’t worry about any stat other than Magic. Willpower you can refill with mana potions and your spells take care of the rest. Rogues meet Strength and Cunning minimum requirements then pump everything into either Desterity or Cunning, or split them evenly, depending on whether you’re a dual wield dagger DPS kinda guy or an archer. Warriors focus on pumping Strength up to a minimum requirement then pump everything else into Dexterity. Don’t worry about Constitution as high Dexterity will make you dodge lots of attack and you still have potions/spells for health.
In my first play-through I didn’t realize the other stats per class are useless and filling them up only wastes points and takes them away from the important stats for each class. Do the mage tower ASAP to get permanent stat boosts, then the werewolf quests for the Juggernaut armour, then dwarf up to the mountains and unlock the dragon, kill it and get the ageless sword.
Again, these are just basic tips. Anyone with any DA:O experience already knows this, but it helps the first timers.