You will more than likely pop this through natural gameplay.
The bull is a easily identifiable. Its one of the bigger Z's that is dressed in a SWAT like uniform.
The Bull charges up for a few seconds and runs at you full speed to perform a takedown on you. To pop this achievement you have to kill him whilst he is charging up.
I would recommend putting the game on easy and having RPG's/explosive ready if you are struggling to get this.