Just a small comment I'd like to add:
You can get this achievement with a buddy, as long as they quit out (you must be host). My friend and I helped each other during the shorter mission (we did mission 3), and he quit right after the last crawler was dead, and I got the solo achievement.
Just to make sure it worked, we did the same when strategy where he hosted, I quit near the end, and he also got the achievement.
EDIT: Please comment on how I can improve my solution before voting negative. thanks