When you reach the Eastern Wastelands you'll be tasked to talk to the Mayor of Subway Town.
He will give you a mission to proof yourself and he will task you to take care of the Mutant issue in the Blue Line tunnels.
Head down the tunnel and talk to the worker. He will task you to leave to the Wastelands and enter through another tunnel.
Enemies: Mutants, Krakens and Big Mutants (the Grenade Launcher wielding ones)
You'll be tasked to grab TNT and a Detonator throughout the Mutant infested tunnels.
You'll get the achievement when your on your way back to Subway Town.