This is very easy to get and very early in the game to achieve. After the interrogation with the Ice King, Finn and Jake will run into rogue banana guards (4 of them). With The single attacker’s, spread them out until two things happen: the Ice Sword becomes available and the banana guard’s health is around 20 HP (they have 60 HP to start with). With patients and well placed hits from Jake - using the denfensive shield as a “pass” for Jake’s turn, Finn and build up and use the Ice Sword to overkill each one of the banana guards.
Very controlled and easy fight to get the overkill shots on.
NOTE: I can confirm the Mother Varmit fight will yield a 4 overkill shot with Jake’s boomerang special attack (make sure to put points into building up the attack for better damage) once the mother spawns her babies into the fight.
Happy achievement hunting :D