I'd like to offer up a similar solution to Epilepsy Sucks', although I think mine is easier to setup and maintain.
Start up a second controller and load forge on Skyward in H2. Turn on infinite ammo or bottomless clip, and change the starting weapons to BRs and Magnums. Turn shields off and turn health to 10%. Turn respawn to instant.
You'll start on a very small floating platform with two initial spawns and one respawn point. Now, you have a couple options. First, you can just stand in front of the respawn point and headshot controller #2. The problem is that the weapons and bodies collect and eventually will crash the game.
To mitigate that problem, build a crate a few feet off the edge of the platform and set its physics to phased or fixed. That way it won't drop when you let it go. Then move the respawn point on top of the crate you just built and set its physics to phased. This way, when you kill controller #2, the bodies and weapons simply fall into the sky below and disappear, removing the crashing and lag issues.
Assuming you start with zero headshots and don't miss a headshot the entire time, you'll need to reload your magnum 209 times.
User Dizyne suggests that you dual-wield magnums and alternate firing them in order to never have to wait for a reload. Could potentially shave several minutes off of this grind.
Hope my solution helps!