Untouchable Achievement in Mass Effect 3
Escape a Reaper in the galaxy map.
How to unlock Untouchable
The galaxy map is located on the Normandy and is required for space travel. While in this view, you have the ability to fire a sensor pulse (see "Lost and Found") which detects War Assets for you. Reapers (an alien enemy) have a chance to pick up on this signal. You will notice a "Reaper Alertness" bar on the bottom left of your screen. After it gets full Reaper vessels will fly after you. They are faster and it will be game over if they get on you.
The achievement requires you to get the alertness up full, and then go through the Mass Relay to escape. The best thing to do is save before trying. Get your alertness up and escape. Afterwards, just reload. An extremely easy way is to follow the route in the video which is extremely short. You can get this achievement in a minute or two.