Off the top of my head getting the Scarab Gun in Halo 2 took me a while. If I recall it involved a frame perfect glitch where you board an enemy banshee right where the level switches in Metropolis. I had to get the thing to follow me through the tunnel, not get killed by my teammates, unstick it when it got hung up on geometry, and not board it before the load, as it would despawn if you just tried to fly it through the tunnel. I think I did it when I was home sick from school. Obviously a great use of an afternoon.
Although I liked the first more than the sequel, Red Orchestra 2 is a fantastic game if you're willing to spend some time getting used to it. It's pretty unique and brutal game that is pretty tough on new players. Adjusting your play style around concealment and cover can be tricky for some people (as you die in one hit most of the time), but is really rewarding once you get it down.