Just get to level 50 however you can in any mode using continue feature that was added as an update. Level 50 boss is kind of hard, but there is a glitch.
Start co-op game and just don't press anything with both controllers. For some reason, occasionally the boss will kill off one of the tanks and stop shooting. Apparently, it seems to think that it's in single player mode, and has done the job. At this point, grab the controller that is still alive and kill off the boss at your leisure.
Do this in each of the mode, respective to the achievement, to finish the game. Progress is shared between modes, so you only need to play level 1-49 once and then can continue on level 50 in all modes.
Here's video version of the above: