From Halopedia, the Halo wiki
It is worth 30 Gamerscore.
One solution would be to make a 1v1 match. It's best to use the Reinforcements gametype. If you're using any other gametype, you should increase the difficulty so they train more units. Next, pick the Arbiter. The enemy can be anyone you want, since it's a Reinforcements gametype. When the match starts, build a Temple and have the rest of the slots occupied by Warehouses. All you should do now is just keep researching the Arbiter's skills and capturing the bases closest to you. The bases you capture should all have only Warehouses built on them. Once you have all the upgrades for the Arbiter and a fair amount of Warehouses, keep all your reinforcements at your base for protection and uses Arbiter's Rage. You should have a good enough income to keep the rage on for the whole match. Once it's on, just keep running around the map killing everything you can. Remember to not get out of Rage Mode.
Another method to this achievement is much easier and faster. The game should be a 1v1 skirmish match with you using Arbiter, and the A.I. using a UNSC leader set to normal difficulty. For this method to work you must be in deathmatch. The first thing to build is your temple. Fill the rest of the spaces with shield generators in case you are attacked while you are gone. By the time you get the achievement you should only be attacked once or less. As soon as Arbiter is out, send him to the first extra reactor in rage mode. Destroy all the enemies there and continue to the next reactor on the map. Since you are playing on deathmatch, you already start with a ton of resources and an upgraded leader. Destroy all the units located at all of the extra reactors scattered on the map and then head to your enemy's base. If you did this quick enough, he should have only a hand full of units, but that is all you need for this achievement.