Interestingly, iirc, I did use Mystea for her stop protection, with Basch as tank.
The thing with Mystea that kinda hampers her is, I think, her equipment. Her kit is pretty damn solid, but she can't do elemental protection as well as Basch, who has more equips at his disposal.
Yeah, I was probably too worried that Carbunkle would be a jerk and target someone other than who he was supposed to with insta-death, and so I just wanted a provoker with Genji Shield to guarantee safety. Should've just swapped the provoker out for Basch and gone the route you did. I may still if I feel like getting that 4* ticket, because for some stupid reason despite Tornado being part of the chain that killed him, it didn't give me the magic kill mission.
Really, the frustration is more about how everyone talked her up as the most important unit to get when she came out, how vital it was to have her for clearing content, and how impossible any magic-based content would be without her. While I get that, and I for sure would probably appreciate her more if I didn't have Basch, I have been so underwhelmed with her that I just can't understand the hype. Trials that she was supposed to be used for almost kill my Basch, and if he could barely take those hits, what chance would she have had?
What prep and what trials?
My Mystea did fine even during Malboro since I fattened her DEF.
===== Her shortcomings comes in terms of HP and SPR if you compare with others since you need equip materias for some fights. (But 30-40%+ mitigation can elevate it somewhat.) ---