Teenage Mutant Ninja Turtles: Shredder's Revenge

Team Attacks happened automatically depending on precise situations you're in. The main one you'll likely see when playing in co-op is that if you and a teammate are standing on opposite sides of an enemy and attacking them at the same time, your characters will do a shell-bump attack, smashing the enemy between the two of you. If you're playing through the game in co-op, this will assuredly come naturally, as it did for me by the halfway point of the game without trying nor really knowing what triggered them.

If you need to force it or go for this achievement specifically, with a second controller you can try to get on both sides of an enemy and press  on both controllers at the same time. It seems a little random when exactly it'll trigger, but that's the quickest and easiest way to grind out this achievement.