I think scaling health and damage is necessary in most cases. If you can kill enemies in a single hit, there really isn't much need for tactics or strategy. Tougher and more dangerous enemies force you to play better and smarter. The best difficulty modes change game mechanics in addition to health/damage scaling and DA2's nightmare difficulty does that. Unfortunately, the combat encounters weren't designed for it, turning stupid design decisions (like waves of enemies spawning out of thin air) into frustrating ones.