Yes listening to the "thunder sounds" for each of the 3 charges on reaping is a good way to keep attention on the screen instead of the bar, and yea 2 seems to be the trick when unstoppable. I do have an issue though, which I am not sure is a bug or what, but I notice at times even with full charges, I hit for nearly…
She can be tough, but mostly due to annoying self healing, and if she is allowed to dispell the cleric shield in phase 2. The most efficient strategy, assuming you have competent DPS, is to gather adds near her, and nuke it all. Done it many times solo healing it on my cleric just fine, also done anything but epic depths…