For that i just had to grind stage 3-4 until my character were strong enough, if anyone is having difficulty with stage 3-3, just fly some units over everything and kill the archer, killing him ends the stage.
By the way, since i figured some people would be curious. I've found the 'Nude' models from the following stages, difficulty doesn't seem to be a factor.
Lesser succubus - Defeat every enenmy and then the last one on stage 3-3 (can be difficult until much later)
Sorceror's Apprentice - Defeat the 'Nude' man before he escapes off of the map.
Succubus - Choose the 'Fail' book and defeat all 3 super mages.
Harpy - I just randomly got this by killing everything between Stage 5-1 and 5-4, it should be noted that stage 5-4 is easy as hell, just kill all of the mages repeatedly before you kill the priest, each one drops a chest (In normal)
Dryad - Defeat almost everything in stage 5-5, i got it multiple times from different enemies, probably the chocobo mounted riders.
Lamia - Still looking for this one, but i'd imagine it has to be between stages 4 and 5 somewhere, but probably 5 since thats where the harpy and the dryad drop. If anyone can figure out where this one is, i'd greatly appreciate it.
Once again, once you get to stage 5-4, you can just farm it repeatedly until you are maxed on all character and have enough money to buy anything, prior to that stage 3-4.
My line up is defense dryad (flex between air attack for casters, ambush for pesky backline shit or roots for aoe knockdown), lesser succubus (drain), Succubus (drain), support Lamia (with poison field), Attack Harpy (with song notes or loving to lock down boss of 4-5) and Attack Sorcerer's apprentice with throw bottle (This spot can be experimented with).
Fun game, one of the best one's i've played in a while.