From Halopedia, the Halo wiki
This article is about the Halo Wars achievement. For the Halo 3, Halo: Reach and Halo 4 medal, see Killjoy.
It is worth 25 Gamerscore.
The player can get this achievement if the player disrupt their own leader power (e.g. using Sergeant Forge), drop a disruptor then try and use the carpet bomb within the radius of the disruptor 5 times. If the achievement does not unlock, position the carpet bomb run just outside the disruptor's range so the resulting bomb run would be within the disruptor's field hence it is still blocked.
Start a game on Normal difficulty, Standard Skirmish, against the Brute Chieftain. Build up your base as quickly as possible. In test; Reactor, 2× Supply Pads (upgrade as quick as possible if not playing as Forge), Barracks and Armory adding two more supply pads if time allows for a base upgrade and turrets at the front two placings. Make a small force of a few warthogs (upgraded with the Gunner) as minor defense but the Chieftain should come running to attack you. Hopefully you should have enough for a single disruption bomb by the time he gets over to your base. Let him attack your forces and drop a disruption bomb on top of him. Leave it a moment and the Killjoy achievement should unlock due to the Chieftain wielding his Gravity Hammer.