Killtrocity

The Killtrocity Medal is awarded for 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. When someone gets this medal the announcer will shout "Killtrocity!" to the player that received this medal.

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.