This is a short chapter and only contains two boss fights, Rezlus and the Dragon. To kill Rezlus you'll just need to put everything you have into him, his defences are pretty high so you might struggle a little. Don't waste time firing the catapult, it never hits him and the mages can easily down you whilst you're controlling it.
To kill the Dragon you'll need to keep attack both his legs, once they have no health left his head will lower onto the ground. Now go over to the catapults and fire at him, you'll most likely land 2 hits (300 damage) and you''ll have to repeat the process again. Once you've done this 3-4 times the fire around his feet will disappear for a short while, and you'll be able to get to Rezlus. Run up to him and hit and you'll see a cutscene.