here is my Solution for this Achievement. Load DNA Sequence 2-2 and directly at the Start you can earn this.
Shoot the Guy, climb the little Mountain and take a Weapon, jump down and you have 4 Targets with 2 Guys in a Row.
Go behind this 2, Push X for a assassinatio Kill and the shoot comes automatically.
If you failed, reload the Checkpoint and you starts at the little Mountain.
PS: More AC3 Videoguide found you here - http://www.youtube.com/playlist?list=PLuGZAFj5iqHcNAlGL214X0...