As above, using both a media II and cure III on flatten is inefficient and absolutely not required on phase 1.

Personally, when phase 1 megaflare starts, I position myself my accordingly (Either in the stack if I have the marker, or to the side if I don't) and start casting medica II as I hear the second set of explosions go off, your medica II should hit right as the markers explode on the stack leaving you just enough of a window to squeeze a potential cure 1 in before the breath hits leading up to flatten.

For phase 4 I initially handled it a little differently due to wanting to play it safe, precasting medica II to be ticking as the megaflare markers popped, then timing a medica 1 to land between the third megaflare hit and the towers. Inefficient but safer if people are short on mdef/hp.