He was probably going for the Vengeful Medal points. Actually, that's probably a great playlist to go for that in since you're only facing one other person, so it's narrowed down for you if you keep getting killed by them.
I already have all of the Gears of War 2 achievements, and I'd need to install it again. If you need an extra player, I'll help, just let me know in advance so I can free up space on my hard drive and download the maps. I'm certain there's people on this site who don't have all 1750. Hopefully they'd join up to get their missing achievements, rank up, and/or add to their Seriously 2.0 count.
I prefer Horde mode, suck at competitive, but if you need help for those achievements I'll do my best.