The ideal way of getting this achievement (faster than killing flood) would be to load up a game of Forge on any map...
In the settings, turn on Bottomless Clip, turn off teams, make the Rocket Launcher your primary weapon and set up as many secondary controllers as you can (You can even get some friends online to come in to make it faster. The more, the merrier). While in monitor mode, pick up a respawn point (Respawn points are the white hologram things on the ground that look like life preservers. Make sure it's the kind that doesn't have a spartan hologram above it) and press
, choose 'Delete All Of These'. Find an open area on the map and place one respawn point for every additional controller you have (So if you only have two additional controllers, set up two respawn points. If you have fifteen players helping you, set up fifteen respawn points). You want to place them so they are grouped together, without overlapping. Have all the other players kill themselves and they will respawn at the respawn points. Place a block in the air for you to stand on. Stand on it and fire rockets at the enemies. They respawn instantly and you don't have to reload. With a turbo-controller, you can get this by having the right trigger fire for a while. You will get one kill for every extra player, per second. So, fifteen players = fifteen kills per second. You should unlock the achievement after a long while of rocket farming.
To add to this, you can make your opponents play as elites. The elite kills will count towards these two achievements:
It will also count towards this achievement, regardless of you playing against elites or not: