Way I did this:
Load up casual dust with bots
Pick CT so enemies will be using the water gun, I mean Glock.
Buy a Deagle.
Run to mid and try to get damaged by all T's
After a T damages you kill them with your Deagle (You don't need to get the kill for achievement but helps if you are not shot multiple times by single bot)
Wait till the round is over not just that all enemies are dead.
If failed repeat.
NOTE: For me the bots only come mid first round that's why I restarted after every try.
Took between 5-10 tries to get the achievement.