Ive also put in some work to figure out overkill, the formula seems to work as such:

if the killing blow minus the monster’s remaining hp exceeds the

monsters maximum hp by a certain % you trigger overkill up to 255%.

Three examples:

monster has 40 hp you hit for 80 you overkill like 100%?

monster has 40 hp and you hit it to 39 then hit it for 81 you overkill for 200%

monster has 40 hp and you hit it for 39 then hit it for 121 you overkill for 255%

my overkill %s might be slightly off how the game says it but thats

the basic function. when you over kill you get a exp multiplier.

The exp is increased by the overkill %. To clarify that is BONUS exp,

you still get normal exp. So for 255% overkill you get 355% exp.

Thanks to WatchGintama for max overkill % and exp rate.

EDIT sometimes it seems to not trigger but I can not tell why, it seems rare and inconsistent…