The part in question is in the very beginning of the game, shortly after
You'll come upon a room that quickly fills with gas. Grapple up to the top and go through the grate into the room. There are two guards and a Joker henchman to rescue.
The first guard is hanging from the platform you're currently standing on near the edge. After rescuing him, run and jump to the next platform and look up to find another platform to grapple onto. The second guard is hanging from this platform. From here, jump and glide down to the next platform where you'll find a Joker Henchman. Rescue him and the trophy will unlock.
This trophy is missable so if you don't rescue them the first time around, you're gonna have to restart the game to get it. The three people you have to rescue are pretty vocal about it so unless you blatantly ignore them, you shouldn't have to worry too much about missing it.