Gears of War 3 - Horde Command Pack

The Host With the Most - host a private Horde Match with a party of 5 players on any Horde Command Pack map - Get a group of 4 other people (or two others with extra controllers) and play through a single wave. Only the host gets this achievement so you'll have to rotate. Do this on a boss wave, on hardcore, with all onyx guards to get the "Kill Locust (Like a Boss)" achievement as well. If you just want this achievement, you can play on any difficulty

Places to See, People to Destroy - host a private Beast Match with a party of 5 players on any Horde Command Pack map - Get a group of 4 other people (or two others with extra controllers) and play through a single wave of Beast. Only the host gets this achievement so you'll have to rotate. You can do this on any difficulty.

What Does This Button Do? - get 500 Silverback rocket kills in Horde - If you can get a hold of a silverback with rockets then simply plant the Silverback with A and start shooting with the Right Trigger. Get 500 kills and you're done. Otherwise, spend $120,000 on wires to unlock the silverback, spend $500,000 on the silverback to reach level 3 and then buy a silverback and upgrade it to get a silverback with rockets.

It's Hammer Time! - achieve Level 4 in Horde Command Center Fortifications - You have to put $360,000 into your turret skill and then another $40,000 into the Command center to get to level 4. After that, upgrade the Command Center all the way to unlock this achievement.

Kill Locust (Like a Boss) - defeat a Boss Wave as 5 Onyx Guards on Hardcore - You must defeat any Boss wave in Horde mode with all 5 people dressed as Onyx Guards. You'll have to do this in a private match since matchmade games are only on Normal at the moment.