In the front entrance you can pin in all the knights if you just put someone in the doorway. this way you can attack the knights one by one with two people (the person in the doorway [which is usually that guest guy by default cause he just charges in there] and i like to use an archer from afar. I also have a chemist in there to revive whathisface when he dies) Of course i often die beforei can finish my onslaught.
Go back to areas like Mandalia Plains, and the Sweegy Woods for the love of god and level up. I was at level 18 before taking on that battle.
Also, a blackmage/wizard would be of great assistance.