One Last Grind
Written by MSG Commander
Here's my recommendations:
Strengthen Kline and Vil's armor to Gigant Mail +3
Strengthen Alia's armor to Dragon Vest +3
Strentgthen Anne and Fred's armor to Dragon Scale +3 (or you can go for Holy King's Armor if you don't want to farm that much Dragon Hide)
Strengthen Paola's armor to Moonlight Robe, Holy Robe, or Dark Robe - just be sure whichever one you choose is +3
Strengthen Kline's weapon to Ragnarok (+ as high as you're willing to go)
Strengthen Vil's weapon to Brionac (+ whatever)
Strengthen Fred's weapon to Moonbeam Sword or Falcon (+ whatever)
You don't need to worry too much about the girl's weapons, as Alia and Paola will mostly use magic-based skills, and you probably won't use Anne that much.
If you have any witch's requests that you still haven't completed, do them now - except for the White Pancho (you'll probably need to be more like Level 90 before you can beat him).
Make sure you have All-Reflect, and give it to Fred for the final battle.
Get all of the boys' attack skills as high as you can (most of them should already be mastered).
Strengthen all healing skills (Energy Flow, Grand Heal, etc., as well as Resurrection and All-Cure Full.)
Also, try your best to max out Holy Breath and Crusade - you will use these skills exclusively in the final battle.
Accessories and Magic Meteorites
The best choice for the coming battle is to use Kline, Alia, and Fred. Kline will be your main attacker, Alia will use attack and healing magic, and Fred will quite simple use All-Reflect on every turn.
Equip one of them with Goddess Eye, and the other two with Goddess Amulet (it doesn't really matter who gets what).
Equip Fred with Rebounding Shield Stone (and make sure you've unlocked All-Reflect, or this strategy just won't work.)
Also, I recommend you buy 3 more Feather Amulets, and equip a total of 3 amulets each to Rooselevy, Letz, and Regius (and make sure to set them all to Guidance.)