When you first get the chance to transmute the squeaky hammer weapon (yellow handle, red hammer) you can get unlimited experience points. Just transmute it and then run up to the nearest enemy and mash away. You're combo meter will be incredible since you can juggle the enemy forever. The experience bonus caps at 480 exp for a 50+ hit combo. So if you want to be efficient: juggle for 50 hits, let the enemy drop, get the combo bonus, repeat.