No, it's too high to grenade jump up to. And absolutely not would a gravity hammer get you up there.
The answers were to Brute Shot jump, meaning you take a Brute Shot, aim it at the ground and once you jump up, fire three consecutive shots while crouching. It only works at the window closest to where the Shotgun spawns I believe.
Another answer was to have a teammate help you. Simply stand on the teammate's head, he jumps and then you crouch jump up. Probably at the same spot as stated above, but I'm not sure if this tactic could be used elsewhere.
a double shot hitting right trigger twice and then hitting x with perfect timing (halo 2 only)