Ashera: Godlike. Not sure if I allow the Auras, but...she's still one without them. High defenses, high offense hitting both defenses, some status on the side, and counters. Only really fears MT ITD.
Caineghis: Low Godlike. The only things he doesn't have is high status defense, weakness to Fire, and gets walled by Anti-Physical tricks (...the extreme kinds. He kind of mocks raw defense.)
Elincia Riddell Crimea (FEs?): Middle/Heavy. Good offense, paired with counters and Pseudo Healing, she can hold her own well enough, though lowish durability.
Kieran (FEs): Middle. He's basically Axe Oscar in FE9...yeah, maybe that's a cheat of a ranking, but still.
Lekain: Heavy. Just a typical good ball-lightning slugger of a boss.
Marcia (FEs): High Middle. Fast enough to double things, Stun is good, and has some evasion.
Micaiah: Middle. I allow her Nosferatu, so she can stay alive better and such.
Nailah: Heavy/Godlike. Caineghis- for the most part.
Oliver (FEs): Abstain. Mostly cause I don't remember his FE9 boss form; may rate later.
Sanaki Kirsch Altina: Light/Middle. Good offense, Rudol Gem and counters, and potential Flare saves her from being a shitty light.
Sephiran: Heavy/Godlike. Hey look, its Holy Elemental Nergal, with actual speed, random ITD, the ability to ignore counters, Instant Death, Critical hit *AND* Random Skill Activation Immunity and High Regen! ...so I guess he's not really anything LIKE Nergal <_<
Sothe (FEs?): Middle. Good evasion and speed, not much else though.
Zelgius (FEs?): High Heavy. Going off his boss form, which is a huge tank with good damage, counters, and a good chance of MUDRERING YOU DEAD should you try to Melee him.
And I agree with Ciato that Zelgius' form in FE9 is so not legal. It has plot armor bullshit, can be skipped entirely (game even encourages you to do so if winning seems unlikely), has a time limit, etc.
The FE10 form is 100% legit, just has some scaling arguments due to the duel nature, but really, that's no different than any other Duel boss we have ranked.