Seifer is stronger then you seem to think he is.
After all he did beat Odin at his own game (1 shot kill)!
His Attack Gunblade swipe,Fira, Fire swipe, Firaga, Fire, Dispel, Haste,Thundara, Tornado Sword attack,Cura, Fire-all cross-slash
Cross Sword Limit Breaks: No Mercy

If odin get chopped by Seifer in 1 hit I highly doubt a Aeon would be much more challenging.

Seifer would cast haste on himself and quickly attack whit his gunblade normal attack and gunblade special attack(Gunblade swipe,Tornado Sword,Fire-all cross-slash). Seymour can always try to cast protective spell on him but only to get dispelled by Seifer. Then Seymour would do attack to just to lower seifer health to end up allowing Seifer to do his Cross Sword : No Mercy then heal himself up while Seymour put peice of his ass back together. The downfall of Seymour will be underestimating the power and speed of seifer.