To unlock this achievement you will have to finish half (5) Co-op Episodes. These are special episodes that require teamwork and working together to finish. You CANNOT play co-op on normal N+ episodes to unlock this achievement.
You can play over XBL or on the same console to unlock this achievement.
This achievement not too bad since you only need to win 5 episodes. It's easy as long as you have a good partner. Team work is the key here, if you don't play well together then you either need to re-think how you play or find someone else.
I noticed that the host seems to have an easier time then the other player. It seemed like the host's tasks to complete the level is easier. I would suggest having the person who has less skill is the person that needs to host.
This achievement is easy to boost, all you need is someone that is willing to boost with you.
The last tip I can give you is that only one player needs to finish the level so look out for opportunities to sacrifice one player to give a good advantage to the other player. Also work together to get enemies to come after you allowing the other player to get through the level.
Overall not too hard but there are several levels that are hard for sure. Keep at it and try not to over stress yourself out! Best of luck!
If you need any help let me know which Episode and I can create a video to upload if needed.