The rage meter build up and depletion of both Garuda and Typhon are ridiculous high, most likely for the simple reason that they are meant to be reprint bosses for a much later date when everyone and their grandmas will be swinging +40k damage per hit even without elemental advantage.
Save Black Prop debuff until you know that she is going to burst next turn for sure.
And use ATK debuff if you definitely know she is going to burst, especially AFTER Black Prop debuff.
One other thing is that its usually better to eat the normal burst instead of the rage burst. Take the normal burst and wait for her ATK Buff timer to run out. Its only like 30 seconds or something which is FAR shorter than any other enemy self-buff in this game.