Again, the units used against you in this battle aren't too tough. There are some snipers starting out directly to the north of you, so you'll need to wipe them out. Also, there are ballistae amongst the Wizard's units. Although they're not great against flyers, they have a phenomenal range, and cause a lot of damage against ground troops. Your best bet is to take them out with Magic from long range.

As soon as you mount the steps, you will be charged by the horsemen to the south and the foot-men to the north, on both sides. They're easy enough to stop. The other enemies in the main room to the north won't attack you until you've attacked them, so get in position first, take out those ballistae, and charge. There's nothing in there to stop you.

It might be wise to let Claret attack the Wizard and his men, if you want her to like you.