You get this achievement for finishing the game the first time. The objective is to defend a series of power cores as they power up. While they're powering up, enemies will attack the cores.
I'd recommend having the Shell upgrade; it makes this sequence much easier.
When the countdown starts, activate the Shell (RB+B) in the middle of the floor, and stay under it and fire at the enemies as they enter the chamber. When the shield runs out, move around to avoid getting hit until it has charged up again.
On Normal difficulty the enemies are those red guys, and they'll fall fairly easily to an upgraded Assault Rifle or Rocket Launcher. Ammo crates regularly appear, so don't worry about running out.
When a core gets damaged, aim at it and tap LB to repair it.
This sequence will take about 4 - 5 minutes to complete. On Normal difficulty I got it on my first try, so it's not hard.