EDIT Aug 26, 2014
: The recent patch allows you to hack/tail your friends. These tails do count towards the achievement, so it's easy to boost now. Just make sure you do NOT scan or kill your friend - they must successfully tail you for it to count. Thanks to those in the comments for confirming this for me.
Patch notes: http://forums.ubi.com/showthread.php/911148-Patch-Notes-08-2...
Old solution below, for those curious what sparked both the love and hate in the comments:
For those who want to force this to happen quickly, here's a pretty good strategy. I don't know why it works, but it does work.
Step 1: Go to options -> online -> turn off invasions. This will RESET your notoriety (MP experience points) to zero. If you don't want to lose your notoriety, don't use this solution - try one of the others.
Step 2: Turn invasions back on.
Step 3: Do an online tail of someone else. It doesn't matter if you win or lose. For me, I wanted to have fun and usually won. In the comments, people are reporting more success if they intentionally lose. Losing is definitely faster.
Step 4: As soon as you're back in your world, you'll get invaded. It happens within 10 seconds without fail. You can tell you're being invaded because if you pause the game, things still keep moving in the background. Most of the time, this will be a hack, and those don't count. Do what you want with these - find them, ignore them, whatever.
Sometimes, during step 4, you'll see that you're being invaded (pause menu trick) but you'll NEVER get notified about an attacker. That's what you want. After a few minutes of this the invader will leave and your achievement progress will increment.
Regardless of which invasion you got in step 4, repeat at step 1. Keep doing the off/on/invade/be-invaded rotation until you get the achievement.
I do not understand why this works. I know for a fact that when I tail other people, they get notified that they have to find me. They start running around like crazy, so they must know. Sometimes when I get notified that I'm being invaded, I know I'm being tailed. The game notifies me. Why does this achievement only tick up if you get no notification at all? Who knows. Maybe it's bugged and it only ticks up if the invader quits out early (they can do this without penalty, as long as they haven't started the hack).