If you rescued the poor goblin chap that was going to die on the first floor, you'll find him at the top of the keep with some clues about "Minions". What are minions?! You'll soon find out. And what the hints mean is that they're weak minded and can be affected by the Charm spell (In fact, they're the only ones in the game susceptible to it.)
There rider stands, staff in hand. You know what's going to happen next. The key is to use the proper strategy. If you kill the Minions, Rider will get scared and start hitting you with Level 3 spells. What you want to do it play it safe. Cast charm on the minions (both of them, each time it runs out). The minions attack actually does some damage to Rider, unlike your own attacks which will land a big fat 1. Once the Minions have killed Rider, you can kill them and claim victory in the battle.
Might as well head back and talk to the King. He usually has something useful for you.