This is obviously easier said than done, like most achievements involving the hardest difficulty. The Best advice for this would be to not be greedy, take your time and pick off enemies, one by one. Use cover.
If you have someone to help have them help you. If you are getting low on health with 1 character, playing by yourself, switch to the other character and the computer will heal you. Another tip is to use Backblast, his weapon is insanely powerful. He has a flamethrower shotgun for short range that shoots mortars at long range. All of which do tremendous damage.