I combined two solutions above into one and gave it a try: I succeeded for the 1st attempt.
1) Saint Devourer's:
Sign onto a second controller and have the primary account (the one you're getting the achievement on) join onto the second ship and just use the second controller to finish the level. Best done on first level.
AND
2) Endersgate's:
If you are doing this by yourself, this achievement can only be obtained on levels 2 and 4 (the ones without bosses). Not the easiest achievement to get, but it just takes a little practice dodging.
Local multiplayer, Normal mode, 2 players, always connected to each other immediately.
I began to rush through Level 1, where sometimes I shot with both controller (to accelerate killing the boss), then after the beginning of Level 2 my main account (the to-be "achiever") pushed only X if someone died, the secondary controller led the commando and shot everything.