This is the impact hammer weapon achievement. Requires 15 kills with the impact hammer in a match to get "Jackhammer" and just repeat in 10 matches
Personally, I do the following settings for most of the weapons achievement and just change the mutator for the achievement I am going for.
Host a player Deathmatch
16 player, 1 required, 15 bots
Map: Rising Sun (it's small so all the enemies are in one area)
Set bots to Novice
Set match limit to AT LEAST 15 kills (Duh)
Set match time limit to something greater then 10 minutes (just in case)
Use the weapon replacement mutator, and change all to impact hammer for this one.
Personally I would not use low gravity on this one as the weapon stinks and with low gravity you hit someone and they fly off making you have to chase them down to get the kill. And weapon replacement ensures bots aren't running around killing you with a rocket launcher while you try to pound them with the hammer.