The first and most important step is a workaround, because Epic has since patched this "glitch", but it can still be done if you follow these instructions to the letter. Disconnect from X-Box Live while on the Dashboard (unplug your network cable or your Internet modem) and clear your hard drive's cache (see the Microsoft website if unsure how to do this). Then launch Gears 3 WITHOUT connecting to X-Box Live.

Start a single-player Arcade match on Act 3, Chapter 1 on Insane difficulty with the following mutators on: Super Reload, Enemy Regeneration and No Ammo Pickups. Skip the two cutscenes when you start and you should be in a hangar full of weapon racks. Empty the clip of your rifle completely and be sure to get a Perfect Active Reload, then fill up your ammo from the appropriate weapon racks and head outside, but not far at all. Look up.

There are Reavers flying over your head. Here's the fun part - you can shoot them down, which will increase your score multiplier, and give you a ton of points. Also, they're infinite in supply and it is almost impossible to die during this part of the chapter. Kill as many as you can until you have a 10x (blue) multiplier and as many points as you like (a million? up to you!) before moving on. When you're ready, get your Super Reload flashing again and load up on more ammo at the back, then head to the front gate.

A checkpoint will trigger when you do, and you'll have to defend the front gate. You shouldn't have too much difficulty here - with Super Reload, anything you shoot will die almost instantly, even on Insane. If you start taking any damage, just let go of the Left Trigger and blindfire. Hold this position until the catapult appears, then head back to the highest catwalk on the inner gate.

This part is more crucial. There will be several Savage Grenadiers (some of which grapple up), some on bloodmounts. You have to kill most of these (without dying, if at all possible), and eventually you'll hear a horn blow and there will be 5 waves of Maulers and Grinders coming in the front gate. This is where you have to stop the assault dead. If it gets to the point where you will be unable to stop the maulers breaking in the second gate, quickly pause and restart the game from the last checkpoint, which will be where the catapult broke in the front gate.

When you stop all the Maulers and Grinders coming in the front gate (again, not too hard with Super Reload as long as you keep your head down), the chapter will end at this point, and after a few non-skippable dialogues, the scoreboard will come up. Here's the most important part - the very instant the scoreboard disappears, when the screen goes black but BEFORE you hear the sound denoting the next chapter/cutscene is beginning, you have to pause the game and select Restart from Last Checkpoint. When you do this, you'll be returned to the part with the Grinders and Maulers again. Rinse and repeat until you reach your desired level (you can see your level go up on the scoreboard each time next to your name).

When you're at whatever level you want to be at, the next time the scoreboard comes up, pause and select Return to Menu instead of Last Checkpoint and when you get to the menu, watch the magic happen like it does in this video!

Enjoy!

(if you make it all the way to 100 you can re-up from the main menu after re-launching the game while connected to X-Box Live)