To obtain this trophy, keep in mind that you must reach the next checkpoint before quitting out for the kills to count. To deflect a Bitch Baby's acid, wait for one to start producing a green liquid and get ready to press and hold X (while locked on) as soon as it spits; one deflection will kill it.
Bitch Babies are encountered throughout Chapter 5, but the best spot to grind this that I found is Dollgirl's Pursuit. After the moving spike pillars, you will enter a hallway that will partially fall apart and will then enter a room where a battle will ensue. There are 3 Bitch Babies here and a checkpoint right after, so just deflect those Bitch Babies' acid (might be easier after killing the other enemies so you don't lock on to the wrong one) and end the fight before reaching the checkpoint and restarting the section until you get the achievement.
If you know of a more efficient place to grind these, please let me know. Good luck!