First of all, it's actually a good thing that monsters lvl up as you do.
Because you don't want to be challenging the same lvl4 monsters that gives you 7expt. Monster LVL UP as you do but you're still WAY stronger your Lvl up than there's, if you had trouble with low lvl monsters at start, you'll find thoses same monsters way too easy to beat at lvl100 not to mension you can junction magic, abilities and ect.
Not getting my point? Go back to the fire cavern at lvl100 and beat some monsters there.
It's like I said...
So, first, you'll need to divide your 16GFs with 6 party members, ''if you hadn't done it before...'' and have thoses GFs Learn most of the Magic Junctions so that you can boost HP,STR,END,MAG,SPR,SPD(If you can), Elm AT, El DEFX4 (prefered), St AT, St DefX4 (prefered).
Hit is your accurancy to hit monsters minus the evasion of the monster.
(Squall is 255% accuancy and Blind decreases it by 100%, so Squall can still hit his enenies even when blinded.)
Selphie can also have 255%
Luck boost your chances of inflicting a critical strike. (Double Damage)
Ultima is good for all characters, except Rinoa (If you're like me, you'll use the Meteor Shower trick of Meteor/Angel Wing. 70k damage garanty long as she's alive)
Metor for all
All Curatif/Protectifs spells to all.
My party was always lvl100.
You should be able to beat the final boss, as long as You defeat some bosses first. Save is useless in battle.
here for all the item drop list