Every so often, there will be one of three special rounds: dodge, challenge (shooting all the spider bots) and assault stages. The game will notify you which one it is as the round is starting.
You don't have to shoot at all during the dodge stages, just dodge the enemies. Stay in the middle of the screen, as the enemies walk in from the sides. If you are on the edge of the screen, sometimes one will walk into you before you even have a chance to dodge. This happened to me a few times. So stay in the center and carefully dodge and the achievement is yours!
Here is a game clip I recorded as an example:
It will say "Perfect!" and you will get a $1,000 bonus.