Normally, the number of nonspells/spell cards for each stage's midboss and boss are like this:
Stage 1 Midboss: 1nonspell, 1spell card(on hard and lunatic only)
Stage 1 Boss: 2nonspells, 2spell cards
Stage 2 Midboss: 1nonspell, 1spell card
Stage 2 Boss: 2nonspells, 3spell cards
Stage 3 Midboss: 1nonspell, 1spell card
Stage 3 Boss: 2/3nonspells, 4spell cards
Stage 4 Midboss: 1nonspell, 0/1spell cards
Stage 4 Boss: 3/4nonspells, 4/5spell cards
Stage 5 Midboss: 1nonspell, 1spell card (with the exception of tewi)
Stage 5 Boss: 4nonspells, 5spell cards
Stage 6 Midboss: 1spell card
Stage 6 Boss: 5nonspells, 6spell cards
Extra Midboss: 3 attacks (1nonspell, 2spell cards OR 3spell cards)
Extra Boss: 8nonspells, 10spell cards
Also, so I guess the enemies would be moogles, chocobos, cactuars, flans, those bombs that explode after 3 hits, etc? XD those are just of the top of my head
And... I don't think Valefor, Ifrit, Ixion, Shiva, Bahamut, Yojimbo, Anima and the Magus Sisters can fit in one stage 5 boss :/