Two-handed swordsmen are murder masters: in a game where enemies tend to cluster, a weapon with a nice wide swing is devastating, and the majority of the moves for a two handed sword user can be upgraded to do critical hits every time they connect, and strike a wide area. And with the skill Second Wind, you can speed up the cool down of your skills, and completely restore your stamina. And you can use it once a minute, or every 45 seconds once it's upgraded. With the proper build, the only reason you need a party with you is to heal you while you reduce every enemy in sight to red ruin.
A dual wielding rogue with the dual weapon and Assassin specialization trees maxed out can easily dispatch boss-rank characters in two or three hits.
A properly-specced archer Rogue gets critical hits for 250% damage close to 70% of the time, knocks back enemies on hits, and will one-shot any non-boss enemies.
That's nothing. A properly built dual weapon rogue can score critical hits one hundred percent of the time. This is exactly as broken as it sounds.
Even better: dual-specializing Assassin and Duelist. You may only focus on one enemy at a time, but that enemy, regardless of rank, is screwed.
Two words: SpiritHealer. Max out the tree, and your character will have a one-hundred percent healing rate, which basically means you regenerate health every second. Throw on a bunch of buffs like Rock Armor, Arcane Shield, and Heroic Aura, and your mage will be taking pitiful amounts of damage at best, damage which will be healed completely moments later. Not only are you the best tank in the game, you can pretty much just turn on auto-attack and cherry-tap everything in the game to death. The cincher? This ability can be gained before Act I is done.
Blood Magic is pretty broken as well if you invest some points into Constitution. After activating it, you can get a Heroic Aura and an Elemental Weapon buff for free. With Aveline, Fenris, and/or Carver hanging about, Improved Sacrifice recovers plenty of HP, and if you've got Anders in the fourth slot focusing on healing he's going to be keeping the tanks in good health as it stands. Grave Robber is a nice supplement for anyone who gets near. Built right, you may now endlessly spam a cycling of Cone of Cold, Firestorm, Tempest, and Fist of the Maker infinitely, taking advantage of the hitstun from Firestorm to Improved Sacrifice Aveline until you run out of enemies.