So far the lime slime is the easiest way to cheese things, but I do find ways of getting more exp is killing bosses that drop boosters, and also yeah, sacrificing stats. I generally just have a go at the companions like giant rat and barrel master, since I can customize their equipment some. And also makign use of that broken sword and ego to bolster exp gains. By giving stats to enemies as well as getting them larger. And upgraded it gives a flat passive EXP boost
There is an exploit, but ya know, same with the slime, easy cheese.