How can I get the 'Can't Touch This!' archievement?

The Raven's seeker missile is not an instant attack... it slowly travels towards its target and when it reaches it, it blows up. To get the achievement, you have to run away from the missile until it runs out of gas so to speak. They last 15 seconds, so if you can keep away that long, you will earn this achievement. You will need a fast-moving unit to do this.

The reapers are a great unit to use for dodging the seeker missile as they are quite quick, even faster with the speed boost.

This is a "Melee" achievement, so make sure your game is not custom, but Melee instead. Also you cannot dodge a friendly seeker missile to get this achievement. It has to be an opponent's seeker missile.

Easiest way to do this one is to play as Zerg and have a speedling on creep. You really don't have to try to outrun if you play it that way.