THIS ACHIEVEMENT IS PRESUMABLY GLITCHED OR AT LEAST VERY BUGGY IF IT DOESNT POP RETROACTIVELY FOR YOU! THIS SOLUTION IS A WARNING
The first red flag for me was when the co op achievements didnt pop even though I already completed the achievements on the X1. Then after grinding kills using the X1 solution for 4 hours, I checked Halo waypoint and saw I had 38k kills. Now I only got maybe 500 kills in the campaign as the achievements were retroactive from the x1/win8 versions of game for me. The same thing happened to my partner who was playing this as his only "spartan assault version". I assume the only way ppl are getting this nowadays is through a retroactive pop. I just wanted to warn everybody. i will update this if things change.
In case you still want to try this, have one person open the door at the start on the right on mission 5 and have a player get the incendiary grenades then teleport back. The other player uses his one grenade on the flood spawn as he steps on the switch to hold them back and then the other player uses his 4 incendiaries spaced apart. There are 3 more fire grenades on the bottom right. You will average 500 kills a game.
EDIT: I messaged the last couple people to unlock the achievement and it seems that the majority of them didn't restart the level ever. It seems that restarting doesn't count the kills towards the achievement, just the waypoint. I'm assuming this at least. Try killing until the time runs out or u die instead
Edit 2: Kovy88 says if it doesnt pop retroactively try playing mission 1 on the xbox one of the single player then coming back to try mission 1 on co op in the 360 version. This didnt work for me but it may for you guys