Shell will halve the chance that Instant Death will occur since that is a status effect. If you worry about MP consumption, equip a Shell Shield the moment a creature readies this attack to increase your chances of surviving. In any situation, you should always have Ally: KO à Arise/Raise/Phoenix Down set up as a gambit with the highest priority for all characters.

Oversouls/Helveniks are weak against Holy, so equip your characters with Holy Lances or even better - the mighty Excalibur if you have it. Like others have suggested with the Baknamies, have Foe weak: ice à Blizzaga set up and just sit back and watch your team tear them apart with it. For Elvorets, Foe weak: wind à Aeroga works very well against them too. If the spellcasters have Faith equipped, there's no doubt in my mind that you'll be dealing 9999 damage as long as those harmed don't have Shell enabled. Doing high damage will also get you plentiful MP back, and since this place is heavy in Mist, shortage of MP shouldn't be an issue. It will only come to that when an Oversoul successfully casts Annul on one of your characters, just have the victim Charge a decent portion back.