Below is a list of achievements/medals that don’t exist for various reasons
Perfectionist - Get 3 perfect kills with the BR or DMR without reloading between them.
Do Not Disturb - Kill 3 enemy Spartans successively with a shotgun while reloading.
Order in the Court - Kill 10 enemy Spartans in Grifball with the gravity hammer in one life.
Volkswagen Approved - Kill 7 enemy Spartans with the fuel rod cannon in one life. (Just for kicks, not a suggestion)
Gun Control - Kill 8 enemy Spartans with a scout warthog.
When Pigs Fly - Have 3 seconds of air-time with a warthog.
Unusual Goose - Have 3 seconds of air-time with a mongoose.
Out of the Park - Throw the ball out of bounds in grifball.
Who you Gonna Call? - Destroy a ghost, a wraith, and a banshee in one game.
Sharing, not Caring - Hijack a vehicle which was originally yours and kill its occupant.
Chauffeur - Drive your team’s flag carrier from the enemy base to your base 3 times in a game of CTF.