ZePedroPONTO: I am on the latest version and I cannot get the "Hot Pot" achievement to trigger, although I have fulfilled the requirement multiple times. I have not yet attempted the similar "Meal for a King".
nervemarkacid: You've shot the fireball into the pot hole and caused her to play the "stun" animation where she is on the floor on fire?
Yes, I was sure I had it because of the special animation. At some point I imagined that it might trigger at the end of the fight, but it did not.
EDIT: In case this helps: I had already beaten the game 100% before achievements were added, so today I decided to start a new save file for the Umbrella run, and also went for the few other achievements I had missed (like "Hot Pot" and "Meal for a King"). All the previously earned achievements unlocked as soon as I booted the game. I don't know if the fact that this happened in the same session where I fought Grandma had anything to do with it.