Re: (Nutaku) Milenium War Aigis (English Version)
First of all: Dont attach thumbnails. It causes eye cancer for everyone trying to help and makes it impossible to get any information above what units you have and what state they are in (before CC, after CC, AW), i. e. we don't know their level. Upload images on a side like tinypic or whatever and put them either into a spoiler or post the link only (don't put the big images on a normal post either).
Regarding Cornelia: I'd feed at least one copy in my main one for the guaranteed CR and skill up. With the others: Sell them for RC, feed them to your main copy as well and pray to the demon mistress that it won't be wasted. I wouldn't keep any unit more than twice, most units I wouldn't even keep twice (yes there are some crazy guys, who play with 15 max level AW Aishas or other crazy stuff...). Seeing you whale enough to have lots of black units and the dark knight already, despite your low leveled units, you can probably feed them into your main unit without any problems. Or you can keep her or do whatever you want. About buffing: She doesn't need any buffing. She is extremely good as a tank with ridiculous def values.
As for priorities, the categories are pretty much what razisgosu said. In general I'd advise to concentrate on a core team first and level it up evenly until they are max level CCed (in steps, first max level before CCed, then CC30, CC50, CCmax). After that AW the units first that profit the most of it and bring them back to level 30.
Now for the categories:
UP generation:
Soldiers: You should have two at a high level. Definitely pick Jerome. For the second I'd pick Zenobia over Katie.
Front Guard tactician: You don't have any.
Valkyrie: Can be concidered a duelist as well, can help with UP generation as well though, Dahlia is okay, both Thetis and Clissa are better though...
Healers: Camilla and Iris should be good enough for the beginning. Some people prefer Fedora over Iris, but you don't have her. Marr or Feng Shui Users in general are great, you don't have her though.
Ranged Physical Damage:
You don't have Spica from the trading post. I'd prefer her over Bashira on the archer front (burst versus consistent damage). Most people use two archers right now, with you having both Elizabeth and Anelia I'd probably advise against it though.
Ranged Magical Damage:
Mostly witches, due to their slow. Easy pick: Adele. Good stats and skill, her AW ability is amazing. Only thing I don't like about her is her look. Your second best Witch is Cloris, she's not that good though and most of the time you won't need two witches anyway. Especially when you have Mikoto.
Ranged AoE:
First place is definitely Mikoto, she's really good. Farne is a pretty good unit if you are able to use SUmmoners correctly. As for mages both Garania and Sasha have good stats and good AW abilities. Garania is probably a bit stronger due to a more useful skill, I like Sasha better though, because of her looks xD Marius is a really good replacement for either of them, due to the higher range. In general one of the best male units (next to Jerome and Gellius).
Duelists:
You have to differ between offensive and defensive duelist and magical and physical duelists. Magical duelists are mainly princesses. Sherry (she becomes extremely good with skill awakening, which is not implemented yet) would be the offensive choice and Lilia the defensive choice. Olivie is a bit difficult. Her stats make her an offensive duelist, but with her skill, which makes her invulnerable against physical attacks she can be pretty valuable as a defensive duelist as well (especially when you don't have immortal Karma).
As for physical duelists: There are mainly valkyries and bandits. From your units I'd pick Lyla as an offensive duelist and Cellia as the more defensive option.
Tanks:
Cornelia, Gellius and Deine are all extremely good. Gellius is the most fensive choice and Deine the most offensive choice. Cornelia is a bit more on the offensive side, untill we get her skill awakening, which will make her a good defensive tank.
Therefore the core team I'd concentrate on would be (fat is what I would use where I left the choice, may be based on personal preference and look though xD Thoughest decision were the duelists. I really like Sherry but don't really like Cillia and wouldn't want to go without a defensive duelist):
Jerome + Zenobia
Camilla + Iris
Spica/Bashira + Elizabeth + Anelia
Adele
Mikoto + Farne + Garania/Sasha/Marius
Olivie + Lyla/Cellia + Sherry/Lilia
Cornelia/Deine/Gellius