You must kill 10 enemies with the Antler.
There are 2 places to get this.
One is the Lodge level where you must get rid of the blast points. There are a couple elk heads hanging around, just grab the antler off of it and start killing.
An even quicker, better place to get this is in the Practice Arena of the Bloodshot Fight Club. Do Thugs, one at a time, exotic melee, then molotovs.
You will know when an enemy has an antler when it looks like he is swinging at you with nothing. Kill the enemy, take the antler, and start killing, the achievement will pop eventually, it doesn't take to long.