[WingedDog]

Seraphs are difficult to fight but not impossible.

Awe is countered by high morale.
Fire shield is countered by fire resistance.
Blindness is countered by magic resistance.
So let's take some statues or/and undeads buffed by army of lead/gold as a start.
Seraphs are living beings, and suffer from fatigue, while undeads/statues don't. So ghost grip, rigor mortis and such would make them weaker.
Next, we could lower their stats, so
1) Darkness is your best friend, and again undeads/staturs are unaffected.
2) Spam Earth meld.
Mix all the ingridients together and voila: seraph fricassee, monsieur.

Besides:
Seraphs are astral mages and can simply be magic dueled.
Seraphs could be killed by unresisting spells like life drain.