Touhoumon Lunatic Walkthrough Part 127 - Elite Four Rematch, Agatha
HMima is very hard to kill. If you try to take her out, more often than not you'll be completely overwhelmed by her ally, whoever it is, and she'll probably end up in the red anyway and Agatha will use a Full Restore.
But as she just keeps using Mana Charge, if you leave her alone too long she'll be able to one-shot almost everybody and even going 1-on-2 on her often won't be enough as she crushes everything. That's why I used status effects to disable her while I KOed everybody else.
Note that while the weather is set to "Sunny", the freeze status effect will never activate (I don't think someone frozen will thaw out though). I learned this the hard way, right before HKirisame used Flamethrower on Sakuya :(
I couldn't have Gengetu out at the beginning because she gets KOed by Blazing Star. But Flandre's Intimidate ability lowers HKirisame's attack just enough for Gengetu to survive one :D