This achievement will come by natural story progression and is unmissable.
You will get this for defeating the final boss in the game (who is located in world 5).
I would always recommend doing the boss stages with a buddy in coop since this gives you the opportunity to revive each other if necessary and basically provides you with more hearts (2 players = more hearts :-D ). As always learning the attack patterns is the key to success in all of the boss battles! Luckily they are not too complex. Personally, I found it a good strategy to scatter the two players in the level. That way you avoid being crushed by the same object at the same time.
If you need help at any point within the fight just refer to the video below:
Video credit goes to: Plagueis Xboxlivefr
Hope this helps you guys! :-)
Any improvements? Let me know in the comments!