This is a warning and a suggestion!*** Spoiler - click to reveal ***Anyone who wonder why you don't get the achievement for beating 10 levels in co op when you should its the same thing. After you get 5 co op episodes done you will get an achievement. Then you continue to beat the last 5 and you will not get the achievement for beating all 10 because the first 5 are not completed anymore. Reason for that is because the 5th one did not save because you got an achievement.
Always wait minimum 5 minutes before starting the next episode! (I waited 6 or 7).
I highly recommend to do all 10 episodes in one go.
And in co op you should not quit the game at any time! (In single player you want to do that after each episode).
So after you have done all 10 do the first 5 again. Then you will get the achievement for beating all 10 co op episodes.
Below is some information about the single player that you should read too!
Do the episodes from 25 or so first. Or 20 depending on your skill. But try to keep the first 15 for last or for when you expect to get an achievement for finishing X amount of episodes..
Reason is that each time you get one of the achievement for finishing X different episodes you wont get progress towards that last episode you did.
It is much better to replay the easy episodes than the tough ones.
So each time you are gonna get an achievement do one of the first episodes.
Also after you beat any episode click
to advance to the next episode. Then press start and exit game to the menu. But then wait 5 minutes (I always waited 6 minutes).If you don't wait 5 minutes the game might not save your progress!