The Lower Bridge Ledge involves ledging inside of either of the two lower bridges that span across the center of the Infinity.
- Get a Ghost on top of of the bridge.
- Set the Ghost up so it is adjacent from the portal, facing the wall beside the portal so there is just enough space behind it to throw a plasma grenade.
- While standing on the front of the Ghost, plasma nade behind the Ghost's seat and run backwards so you are between the Ghost and the wall.
- This should bump you into the bridge, and, if done properly, should put you on a ledge inside of the bridge.
- Clear your view and jump onto the ledge in the main section of the bridge.
- Proceed on the ledge to the opposite end of the bridge.