Marvel's Guardians of the Galaxy

Flair Attacks are one of the things the game doesn't tell you about at all - when an enemy gets lower in health, you may see a prompt above their head (it could be faint if you're farther away) that says  + . You need to get in relatively close and press the combination of buttons, and you'll execute a melee finisher with an ally. This is a Flair Attack that counts for this achievement.

Flair Attacks, in my experience, don't seem possible on regular enemies. It always needed to be at least a "medium" enemy or tougher for me to get the prompt above their heads. I routinely got them for the yellow-shirted converted enemies, and I often got the prompt for Inquisitors. As long as you pay attention and take advantage of this prompt each time it pops up (especially since it's an instant kill), you will unlock this achievement long before finishing the game.