I remember in Castlevania: Circle of the Moon for GBA, during the summer of 2001, I got up to Lv 99 by sliding in and out of the last room of the Battle Arena over and over again to fight the Devils there, since they gave a shitload of EXP. I spent whole days doing nothing but killing those same few enemies. Keep in mind this is a game where you're pretty much invincible at Lv. 50 or so.