Who's ready for a glitchy trophy? Don't worry...it's glitched in a good way!
The Iron Lung trophy calls for you to hold your breath while sniping for a cumulative time of 30 minutes. I completed my first playthrough and had maybe 9 minutes or so for this achievement. I found that holding down RT to bring up your scope and then hitting RB to hold your breath was awkward, so I didn't use it that much.
Thankfully though, there is a way to get this and you don't even have to be at your controller. Locate any enemy (on any level) that is UNAWARE of your presence. Scope up on the guys face and let the lead fly. If you get the kill-cam sequence you are in luck! Wait for your bullet to hit the enemy. Once it does, pause your game. The time for holding your breath will keep counting even though the game is paused. Now go take a bathroom break, make a sandwich or maybe you could floss your pitbull. Just let it sit at the pause screen until the trophy pops!