Killimanjaro

The Killimanjaro Medal is awarded for killing 7 opponents within four seconds of each other.

Summary
If you were to get one barely under the time requirements, you would have to get 7 kills in just under half a minute. It is available in all Multiplayer gametypes, and is assumed to have a Bungie difficulty rating of 10 out of 10 since the Killtrocity Medal has a difficulty rating of 9 (out of 10).

It is represented by a platinum circle with 7 yellow-gold stars.

This medal is seen very rarely, and most often in custom gametypes, or in Grifball. When someone gets this medal the announcer will shout "Killimanjaro!" to the player that received this medal. The name is a play on the African mountain, Kilimanjaro.

It is easy to get on Halo 3: ODST's Firefight mode map called Lost Platoon as enemies try to attack your Warthog and you just gun them down. It is recommend to take it to the center structure, so that the Wraiths cannot hit you as easily.

See Killionaire for an easy way to get this medal.