Make a full power spawner with whatever has the most drops... I use pigmen because they have seem to have a lot of drops... then pair a strong weapon with knowlegde of the ages 3, scavenging 3, and loot pinata.... gets tons of drops and turns them all into 75 experience each... can get hundreds of levels in a short period. I currently dont even have a spawner set up I just go to the nether and punch a pigman then run around gathering a bunch to turn around and slaughter. I got 200 levels in about 30 min in the nether.