This is best done in the endgame with Bruno and a Burst+ ring. Find a weak enemy like a slime, transform Bruno and use the Masquerade spell. The first hit will kill the slime and you'll get 11-15 overkill hits. If burst activates, then you are hoping to chain all 3 of his automatic attacks (Dark Spark, Poison Knives & Meteor Fall). This adds another 19 overkills to give you 30-35 easily.
Fair warning tho, the burst is random and getting all three to proc can be frustrating. It took me 45 minutes with an encounter ring to finally get all 3 bursts.