Untouchable

The Untouchable Medal (known as Overkill in Halo 2) is a multiplayer medal awarded for killing 25 opponents in a row without dying. In campaign and firefight it is awarded for killing 50 enemies in a row without dying. It is available in all Multiplayer gametypes. It is assumed to have a Bungie difficulty rating of 10 out of 10 due to the fact that to kill 20 opponents in a row has a difficulty rating of 9.

Tips
One Strategy to get this medal is to use the Banshee. You have to monitor the Spartan Laser(s) and the Missile Pod(s) and make sure they don't end up in the wrong hands. A really clever tactic is to take the Spartan Laser so the enemy team can't get their hands on it for a long time. Even if your team decides to take it, it can still end up in the possession of the opposing team. Also, preforming a Banshee maneuver after strafing an enemy can avoid being Sky-Jacked. Probably the last danger someone may face is the host migration system. If the host leaves the game, the migration system automatically resets everyone's sprees.

Trivia
Earning this medal in Halo 2 will result in the announcer proclaiming "Unfrigginbelievable!", which was eventually turned into a medal in Halo: Reach.