I would recommend doing this right at the start of the game as zombies are really easy to kill with fists. Best way to do this is to jump and kick the zombie to knock them to the ground. Then beat them with your fists. That way they can’t fight back as you’re hitting them making those 25 kills a cake walk. Best done on normal walking zombies.
Sam B is also great for this as his main special ability involves using his fists which tends to be a 1-2 hit kill while under the effect of his ability.