Luckily My Shield Will Protect MeG 25

You need to pick up the power-up that looks like a sphere (bubble) and use it to block any enemy attack 20x. You'll here beeping before the attack, and it'll show you what weapon is heading your way at the bottom of the screen. The achievement will pop as soon as you do your 20th. Should come naturally while doing your 2 career runs, but if not you can finish it up in split screen/free roam with a second controller.