General dark souls rule. If the enemy isn't moving you already missed your heal window. Learn which move made him not move and start to heal right after avoiding that move. You can't be reactive of the current move, you need to predict the next move from the current move given the bosses Moveset.
Great advice
Godskin apostle... That bastard input reads and throws a fireball as soon as you touch the estus.
Don’t some bosses have a specific response to using a flask in some games?
They might, I'm not sure. Elden Ring bosses have been accused of input reading because they seem to know exactly when you're about to drink from your flask; that seems to be most known example in their catalogue.
There are videos on it too. Here's one I watched by Zullie: https://youtu.be/XSbuGGnntRA
