For this achievement you must obtain a 45+ kill streak. There is only one location in the game that can even pass 28 and that is in play 3 act 4. During this act towards the end of the level you must slide down a rope while killing enemies flying on rockets. Once you grab the first taco you can not let any enemy damage you otherwise you will forfeit the achievement. The second taco will give you more time and you have a total of 54 kills if you kill everyone. This achievement is not as hard as it sounds. The enemies spawn in waves. Each group follows in a line so hold your reticule still at the front of the line and they all will run into your bullets.