Once you complete the first T Rex fight, you will get this achievement. Watch the video below for how to beat the T Rex.
For those new to the game, the T Rex WILL OUTRUN YOU, and so you need to find cover under the large tree that's just up ahead and to the left once the cutscene ends at the start of the boss fight. Once in cover, blast away at the T Rex until it drops.
When it does, whip out your knife and head over to the T Rex to begin the subsequent cutscene that ends the boss fight.
Credit to andrescanoferguson for posting the video to youtube.