Gears of War 2

First off, make sure you have the combustible maps downloaded. They are 800 MS points right now, might be cheaper or free later, but I did not wait. The choice is up to you.

Next, you and a friend or a couple of friends, choose the three combustible maps to go through; Flood, Fuel Station and/or Goldrush. You just need to get through the first 10 waves on each map, so I recommend you making the game private and putting it on casual. I did this with my wife and on other times with one other person. You do not need 5 people, though it will make it go by a lot faster.

Finally, just some tips. On Flood, stay in the middle where the Boomshot/Torque and Mortar/Hammer of Dawn spawn. There will be grenades just outsied either door, and you will be able to see where the locust spawn. Easy pickings, you can also jam the doors with boom shields after you kill Maulers in the later waves.

Fuel Station is also easy. You just need to stay near the gas station and not too close to the trucks. They blow up after a few hits. Have someone with the Grinder go upstairs rain down bullets or grab a sniper and shoot from above. Just line the stairs with Frags and you will be good. Of course, that is just for when you get up to waves 7-10.

Gold Rush is just a little bit harder, as there is no place to really hide at and the way the Locust spawn is sporadic. Just keep the group together and be able to back each other up. Not hard, just cover each other to make sure no one gets chainsawed by a Cyclops.

Beat waves 1-10 on these maps Flood, Fuel Station ,Goldrush

private,casual, and there you have it

Alternatively, it's more than possible to do this solo. I happen to love Horde mode so I was all too excited to get achievements for playing it.

When I did it, I had it on "Private Xbox Live" (as opposed to "Local") so I'm unsure if the achievement will pop for doing it on Local. EDIT: Thanks to Glycerin Ghost and Jonamon23 for confirming that this achievement WILL pop if done in the "Local" game mode.

As for tips, put it on Casual difficulty and just play as you would in Campaign. Stay behind cover if you're taking fire, be aware of your surroundings, and make good use of planted grenades to cover yourself.

On wave 10 when the Bloodmounts come, hopefully you have a shield by now, which in my opinion is the best way to survive them. I like the shield + Gorgon pistol combo. Back yourself against a wall and/or a corner, making sure your right side is protected (such as near a wall as well) since your shield is on your left a bit.

When I don't have a shield (or if you die and start from 10), I use the Lancer on the Bloodmount, preferably with Active Reload. They tend to stumble and tear their helmet/mask off after a couple seconds. Use that time to reload (again, preferably Active) and finish it off. The Grenadier will likely spill to the floor, giving you a few seconds to shoot him while he's down/getting up.

Overall, it's not difficult in my opinion if you're used to Horde mode and its patterns of spawn/aggressiveness. Have fun!