There are 46 medals in the game, but you only need 32 medals for this achievement. These are the first 32 listed on the menu under "medals." If you've previously earned this achievement in another version, this one might be buggy. You can check there to see which ones you are missing.
1-9: Double kill through Killionaire: kill 10 enemies within 4 seconds of each other.
10-14: Killing Spree through Untouchable: Kill 20 enemies in a row without being hit.
15, 16, 17: Rifle Spree, Target practice, and Contact: Kill 10 enemies in a row, with a rifle, without being hit (or killing with another weapon). Does not need to be the same rifle. You can get some kills with an assault rifle and some with a plasma rifle.
18, 19, 20: Pistol Spree, Pistol Perfect, and Gunslinger: Kill 10 enemies in a row, with a pistol, without being hit (or killing with another weapon).
21, 22, 23: Grenade Spree, Cluster Bomb, and Let it Rain: Kill 10 enemies in a row, with grenades, without being hit (or killing with another weapon).
24, 25, 26: Splatter Spree, Road Rage, and Sunday Driver: Kill 10 enemies in a row, with a vehicle without being (Significantly) hit. You can use any of the vehicle's weapons for this.
27: Assassin: Kill an enemy with a melee attack from behind. If you walk up to a jackal and keep meleeing it, odds are about 50/50 that it'll count as an assassination.
28: Sentry: Destroy an enemy vehicle with a turret. Easily done on C-3.
29: Anticipation: Kill an enemy with a charged shot (from a plasma pistol). Kill a grunt, take his weapon, shoot his buddy.
30: Demolition: Destroy a structure with a grenade. The second turret you destroy in A-2 is a good target for this. The structure must have had an enemy in it recently to count.
31: Swiss Army: Kill 5 enemy units with 5 different weapons. This doesn't count grenades or vehicles.
32: Wrecking Ball Destroy an enemy vehicle with a vehicle of your own.