Hello folks,
So, I went on my paladin to ST as healer. I was trying to join the queue and then leave it again. Well, it worked (was bored ^_^). Anyway, couple of minutes later I got the achievement "Damage Control".
So basically what you do:
1) Go in an instance
2) Sign up for any BG
3) Leave the BG queue
4) Pewpew all mobs down (or heal your party ;-) )
5) ??
6) PROFIT
This "technique" may need some testing tho. As I said before, my instance was Sunken Temple and I was the healer. I signed up for Alterac Valley and left it.
Cheers
Edit: According to the people who have replied, is the achievement just randomness. I just got a bit lucky with the queue and leaving queue then. Sorry for wasting ya time! ;-)