Triumvirate
Kill the three end bosses in Chapter 5 (Harry Flynn, Eddy Raja, Zoran Lazarevic) within 20 seconds of each other on Crushing

Gladiator
Win 50 matches of TDM

Fly on the Wall
Get 3 Hang In There Medals while on Crushing Difficulty

Deathless
Complete a Co-op Adventure Map on Hard Difficulty without losing a single life

Stealth Master
Get 50 Assassin Medals

Looking for Loot
Get 50 Three Medal Pickup Medals

Long Gun Lover
Get 50 Commando Medals

Double Trouble
Get 50 Double Down Medals

Boom, Headshot!
Get 50 Head Cracker Medals

I'm on a Roll
Get the Unyielding Medal while on Hard Difficulty