Gears of War 3 - Medal Achievements

It's All About the Loot! - earn the Bronze "Loot Courtesan" medal - To do this, you need to complete 10 bonus objectives in Horde mode. 

All for One, One for All - earn the Bronze "Force Multiplier" medal - You must get and maintain a 10x multiplier in Arcade mode 10 times. 

First Among Equals - earn the Silver "Number 1" medal - You must come in first in Arcade mode 60 times to earn this medal. 

Award Winning Tactics - earn an Onyx Medal - For this, you can earn any Onyx medal. I found the fastest was Ruthless in Beast mode. You can easily do this legitimately, just work your way up to Berserker and you can make 50 kills per match (10-20 minutes).

Socialite - earn the Onyx "War Supporter" medal - To do this, you have to participate in 30 Gears Events. Events are any alternate gametypes listed on the Event calendar. Not included in this are announcements of map packs and DLC. There's no easy way around this one, it will likely take you playing at least one match per weekend for 30 weekends to get this.

Seriously 3.0 - You are the ultimate Gear! - What is actually required to earn this achievement is to get all 65 medals to Onyx which includes making it to level 100 but does not include special medals like "Beta Tester" and "Member of the Old Guard."