Fix -Atk +Hp S!Cordelia with Atk+3 Seal?

Also putting her in a cavalry team or in a mixed team with tactics skill would help. You got really unlucky there. You can give her life and death (3) for totally improving her offensive stats or give her fury 3 for more bulk but I'd reccomend life and death.
So with life and death 3 and attack+3 seal she would have (with no weapon) 37atk and 43 speed. If you add the buffs of a cavalry team she'd be pretty much fine.
If you go with life and death, due to a lack of bulk, the firesweep lance, as a weapon, is a must.