Unreal Tournament 3

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.

You can do this with a second controller and no sweat.
Sign in your first and second controller an go to Instant Action.

You'll need 15 kills per Match to get the Jackhammer Award.

Just do the following set up:
Gametype: Duel
Map: Morbius Arena (because it's small)
Splitscreen: Yes
Bots: Doesn't matter since you two all by yourself
Goalscore: 20 (will count to the 100 Matches with 20+ scores in the Paint the Town Red Achievement)
Time Limit: 20 or less
Mutators: Weapon Replacement: Configure all to Jackhammer

Just repeat this in 10 Matches, shouldn't take that long.

P.S. You can work also on the Killjoy Achievement by killing yourself with the Jackhammer after 15 Kills and ending the spree.

Set up a game like shown in the following video, but instead of spawning pistols, spawn the impact hammer and go through killing everyone. Repeat 10 times to get "Jackhammer" instead of "Gunslinger".

Jackhammer is awarded for 15 kills in 1 match with the impact hammer.