the best way to get exp, when you kill the boss get the purple balls and kill urself before it completes the level. you will have to fight the boss again, but you will keep all he exp. you can do it over and over.
well the cleric is really easy, but thats not really what matters to me. U need to have a bit more space for inventory(since it is a dungeon game, your supposed to get a lot of things), and the further i got into the game the more monsters there were i literally couldnt take 1 step without getting attacked by about 3 dragons at the same time, so for that i think you should have a slightly more expensive item, a repel-like item, so that when you are at a high enough level with enough money you dont have to fight as much.