Gears of War 4

This achievement needs no explanation in my personal opinion however, it should be best to pair this with difficulty related achievements such as "Learned the Hard Way" or "The Night is Dark and Full of Terrors." These are for beating the campaign on Hardcore and Insane respectively. Simply beat the game in co-op on any difficulty and you will get it but, there's no sense in not pairing it with some other achievements.

IGN's Insane Difficulty walkthrough should be a good starting point for you and your co-op partner.
https://www.ign.com/wikis/gears-of-war-4/Insane_Difficulty_W...