Your best bet is to hope for a 20x or more event, and then KOTH boost.
Doing it legit at the standard 2x experience is a gigantic waste of time unless you absolutely love this game more than you love life, because that's how much playtime it will take to get it. My entire Gears 2 career I had played TONS but I only cracked level 50 around the end of 2010, which wasn't anywhere near 100. Didn't hit 100 till they had that 31x event around Memorial Day (I think?). Took about 10 hours of KOTH boosting at that big a multiplier (factoring in time lost to losing players to disconnects and refinding each other), so you know what you're in for.
Last edited by CaptFrost; 10-24-2011 at 12:56 AM.