The Killtrocity medal is obtained by killing 6 opponents within 4 seconds of each other.
It is available in all multiplayer gametypes and has a Bungie difficulty rating of 9 out of 10. It is represented by a gold circle with 6 bronze stars. Achievement of this medal is noted by an audible announcement of "Killtrocity!" for the player.
One relatively easy way was to just play the Double EXP Weekend Grifball playlist. You just have to camp where the other team spawn near the back and hammer them when they do. It is possible to achieve consecutive medals using this tactic. However, it is possible to achieve so many of these medals in such a short period of time that it will only finish shouting them to the player after the player is already dead, assuming it shouts them at all.
This medal can also be earned on ODST. Use the Tank and blow up everything as fast as you can and you can get this and higher muti-kill medals. You can also run over large amounts of grunts or shoot them in the head with the handgun in Firefight.