Ugh, sorry for not posting earlier. Got slammed at work and I'm still recovering

Anyway, I did some theoretical thinking and now present to you the GREATEST DEFENSIVE FORMATION OUR TEAM CAN MUSTER.
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Bad guys-->Gr Al Ed Ch An
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Gregor holds the front against incoming foes with his good all-round stats. His Sentinel bonus grants Alexander +5 DEF, while the General Guards Gregor (from behind him somehow, logic be damned). This renders TEAM STEELWHAL virtually immune to physical attacks, and if Alexander uses Pure Water they have reasonable protection from magic as well. Edwin hangs out just behind Alexander (since he's the only healer with no supports, and thus he's as useful here as anywhere else), either using them ailment staffs to help thin the enemy forces or using Call Magic to heal any wounds Alexander takes each phase, and Edwin's natural HP regen means that he can Call Magic more than anyone else on the team. If he has a Soothe staff to deal with nasty status effects, even better. Charlotte hangs out just behind Edwin, just within longbow range of the tiles in front of Gregor for covering fire AND within Support range, boosting Greglotte's overall effectiveness. And finally, we have Anja hanging out behind Charlotte, within Support range of Alexander and capable of boosting Charlotte's effectiveness with performances. And if anyone goes down, healing items can be quickly passed between party members.
Of course, this only works on maps where the enemy forces cannot simply fly or move around, but considering the final battle will likely take place indoors this shouldn't be that big of a problem.
* Sirus knocks on ALL the wood