I did this with two other friends on coop. We were religious about checking headcount at each save point, once you meet up with both teams you should have 6 marines, note that you can always go back to the last checkpoint via the menu if someone dies.
When you get to the point where you put Cortana in the door computer there will be a dead marine on the ground, don't worry about him he can't be saved. Try your best to upgrade the weapons of the marines then get on the stationary guns to kill all the incoming Prometheans (mostly crawlers). Facing away from the door, the middle and right guns should take care of the vast majority of the crawlers. The left gun occasionally has to bat cleanup. I suspect you could do it fairly easily with just one buddy and a bit more patience leaving the left gun closest to Cortana empty.
I feel like this would be an extremely hard achievement to solo. The marine AI is just too stupid and will charge in an soak up bursts from the crawlers almost immediately if you don't kill them with speed. I suppose I would have to recommend the middle gun as the best "all around" vantage point, but it will leave you out of LOS on some targets and you'll need to hop on and off the gun occasionally to hit them.