This achievement will unlock by natural story progression and is unmissable.
You will get this for defeating the boss of the 3. world for the second time.
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 (jump to minute 7 for the boss fight to start)
Video Credit goes to Dark Adonis
Hope this helps :-)
Any improvements? Let me know in the comments!