I got this achievement today, and to be honest... it's way easier than it looks alone... i solo'ed all the levels... Here's my strategy on doing it alone:
The first 4 waves should be very EASY to complete by yourself, If you're really good at Gears of War 2 it'll only be around wave 7 that you (might) wipe. Starting from the 5th or 6th wave, there will be grinders... kill these, get their weapon and shoot everyone into pieces... Multiple grinders will come in 1 wave according to wich wave you are on.
I replaced the shotgun by a torque bow, because this does alot of damage as well and if you aim well you can headshot the big guys with 1 arrow.
For the rest of the mobs i used my normal chainsaw gun... fast reloading gives damage bonuses and all types of Horde will go down without even reloading...
Good luck y'all!