i'd say, as long as the resistance isn't too great (pick a value... say, >50%), it should still target the flying enemy... unless doing so results in a grade penalty. in that case, she should avoid targetting that enemy and target other enemies, because if you REALLY don't want her hitting other enemies, you could then tell her to stop using magic at all. (i assume you could use a convenient command, or something, instead of having to screw with her AI.)
truthfully, it would be better to have the AI be more tweakable... for example, you should be able to specify whether you "prefer" her to target flying enemies, or you "only" want her to target flying enemies. possibly even multiple values in between Prefer and Only. if it's set to "prefer", in this case, she'd target the other guys. if it's set to "only", she'd target the flying enemy if there's no grade penalty and if her TP use is set high enough. if not, she'd just defend. the in-between selections would split these results percentage-wise.
hope that helps in some way
