Your party members are pretty strong this time around so on Normal, at least, I mainly kept in mind my own health and positioning, like for a rogue, behind enemies is the best for his backstab move.
With the boss at the top of the tower, it will be harder. I ended up losing my party and barely got in my last stab to finish him off. And this is on Normal difficulty. So for you guys, I recommend setting an AI command to heal when they need it to keep your party in the game.
Recommended talents to have (for rogues at least) are Dirty Fighting (stun), Deadly Strike (heavy blow), and Below the Belt (knockback).
Don't be afraid if you see your back on fire. The mage, I think, enchants your weapons to Flaming for extra boost. Enjoy it. ;)
TIP: Let loose the dogs from their cages and make use of the ballistas to even the odds a bit if needed. Be careful of the grease trap in the "fire chamber" (you'll see what I mean).
NOTE: Any difficulty above Normal leaves you vulnerable to friendly fire. The mage has area spells that can be detrimental if you get caught. Be careful.