I am playing Diablo (original, NOT DIABLO2), how do I gain extra experience points?

I finish each level of the cathedral, but I want to gain experience levels faster. How do I gain extra experience points? I go back to a level and no monster are there for me to kill. What do I do? At the end of the game I am such Diablo will be hard to kill, how to I gain extra experience points to finish him off?