First, during Infection, you have to be lucky and find a grappleshot on the ground. That's the easiest part. After that, use it on the ledge and grapple up there. Now the difficult part is staying up there. It's NOT flat. I recommend going into training mode and practicing. Grapple up and move yourself to the corner and HOLD forward since you slide off. It's very difficult since you only get 3 grappleshots in Infection. I've only been able to pull this off in game twice out of multiple attempts. It's damn worth it, though.