MechWarrior 5: Mercenaries

You unlock this achievement for completing story mission:

This is a tough mission with weight limit of 400 tons, so bring your 4 best Assault Mechs and pilots.
ACHIEVEMENT UNLOCKS AFTER YOU COMPLETE THE MISSION AND TALK TO YOUR CREW IN YOUR BASE/DROPSHIP.

My own mech setup was Atlas witch 4xTier 4-5 MP Laser, 2xT5 LRM 10, AC20BF T4 and maxed armor. It cuts through assaults mechs like butter. You should try to aim for headshots, in close combat it can take down assault mech in seconds.
If you have really hard time with assult mechs attacking youn on last part of mission, hide behind dropship, mission will end when timer reaches 0.