Hot Flash is the boss of level 9. For the achievement to unlock you just need to defeat the boss without dying. Here is the method I used:
Once again, the point of no return is where you are crossing a bridge and you can see trains moving in the background. Once off the bridge you will fight a small group of enemies. If you are low on health let these guys kill you so you can start the boss fight with full health. If you die during the boss fight the achievement will NOT unlock.
For this boss you need to start by taking out all the goons. They come from the left and right so just stand in the back near the middle fire hydrant and shoot them from a distance. Hot Flash will then start shooting at you so just run back and forth to avoid her flames. You will get prompted to use the fire hydrant so run over to the one that is shooting water and rapidly press to increase the pressure and hit the boss. Repeat this process 2 more times and she’ll be knocked out.