This is as simple as it sounds. When one of your characters is bound by and enemy you need to break that bind by attacking the enemy.
The earliest this can be obtained is chapter 1. The Scorpion Sentinel can bind a character and you can attack it to break them free. It's far easier though to wait for chapter 3 during the sidequest "Nuisance in the Factory", as you'll fight groups of gorgers. When they attack with Feast they bind a character for several seconds and hitting them will break the bind and pop the trophy. I believe it will also trigger if you are bound and your AI teammate breaks you free (but I've not verified this).