Ok,
to do this you probably wanted to already have beaten the Sentinel. In the long hallways leading to him there are a lot of hard monsters that give you a lot of experience. The best monster there is a Wonder Bird. Beating one of them gains you over 8000 exp points!!!
I usually find them in the fourth hallway. You have about a 30% chance of encountering the in that hallway. Unfortunatley, you can only find the wonder birds in the hallways leading up to the Sentinel. If you arent good enough to beat the sentinel yet, there are other types of those birds in magma rock.