I found this achievement to be a bit finicky in that it didn't register every single hit to Jason with the baseball bat - generally only the ones that gave you the 'Fight Back' xp bonus, and the ones which stunned him.
Recommended counselors (high luck = higher weapon durability & stun chance): Chad, Jenny, Vanessa, or Tiffany - also Tommy Jarvis (take advantage of this should you come back as him)
- I'd particularly recommend being a female counselor as you can get/wear/use Pamela Vorhees' sweater - a guaranteed stun
Recommended perks: Slugger (start with a baseball bat), Heavy Hitter (baseball bat stun increase), Man At Arms (increased weapon durability), Sucker Punch (melee stun increase)
You'll want to be trying to stun Jason (so you can hit him while he's stunned), so try and find pocket knives and firecrackers. If camping out in a cabin for him, also try and find one with a baseball bat in (should you be using the Slugger perk), and place a trap in front of the door to also attempt to stun him.
Although this can be done without stunning Jason it makes it a lot easier when he is stunned - be wary that it's easier to be grabbed when in front of Jason, so try and make sure you're to the side or behind him.
Of course this achievement can also be boosted in a private match.