two ways to get this achievement
1) your really good at this game, and you can kill other players alot without dying
2) (boost) start up a private match and get 1-3 buddies to help you
obviously the 2nd method is a lot easier... when i did this (with 1 buddy) i loaded up public assembly and picked Wesker S.T.A.R.S (but if you want you could use any character your most comfortable with). Then when the game starts just meet up with your friend and shoot at him until he is dying; then finished him off with an melee attack. Rinse and repeat this until you got 40k (or a little more just to be safe). Then all you have to do is try not to die and let the time run out. A good spot to hide is on the 3rd floor (roof) of the big building and stand on the edge (where you can jump down). I found out for some reason that if you stand at that spot then only a few enemies (if any) will come at you.
As a side note if you feel like you don't have enough time you can go around smashing time bonuses to give you a few extra minutes.But the few times i did this with friends i found we had more then enough time