Gears of War 2

1 player solution:
Start up horde on casual (sanctuary) and go up in the middle then up the side stairs to the corner with a statue. Up here is 2 entry ways (which blocked by charger-shields is a good strategy) take cover here and be sure to look out for invading enemies. Chargers from the ground level is worst so block that one off first, If one charger comes up the stairs use shotgun and go around the statue while shooting him. Pick up his shield and place it in the stairs (Hold left trigger+A) to place it. Kantus will throw grenades but you can avoid by jumping to a slightly different location. Be sure to kill them tho cause they are very annoying. In between when you feel its safe or near end of a wave go down to middle and pick up the blue ammo box so you have more shots.

Also a tip is to pick up a shield after a wave is complete and wait about 10-15sec into next wave then place it infront one of the steps. I recommended to use it on the down-stairs entry. Also needed for this achievement is level 15 in multiplayer.

An easy way to get this achievement is to go up the area with the stairs and stay there. If you have 5 people split up the group so that 2 players on one side of the stairs while the others hold up on the other side. Just hold out for 20 waves and you are on your way.

The second part of the achievement is to get level 15 in multiplayer. I reccomend something you enjoy so that this achievement seems painless. I reccomend wingman or guardian since it provides a lot of points if you are good enough. Achievement popped.

Well, this is my first guide...
I tried the above solution with the boomshields in the stairways and It did'nt work out for me. What I did find though is this:
On waves 10, 20, 30..... so on (the levels with bloodmounts) If you're having trouble with the mounts the trick is to kill the bloodmount but leave the Grenedier Elite ALIVE, he will generally run away and take cover. After doing this with 3 - 4 bloodmounts your left with a gaggle of G Elites. While they are on the map no more bloodmounts will appear and you can concentrate on mashing grinders or maulers (wichever you find easiest to deal with). I leave the grinders alone and stay clear of them and batter the maulers instead. Occasionally I'll kill an Elite to bring back a bloodmount.

Be careful near levels end though (red bar changes to the number 5) I've come unstuck many a time with the seemingly random selection of baddies thrown in for good measure, a flamer here and a ticker and a wretch there!!!
Good luck