If you received enchant from a sorc, only her facets will help, your necros %fire damage won't help
The fire damage may help the summons after they are enchanted, idk how that works but as far as boosting the actual enchant it's sorc stats only
Enchant if buffed twice.
First by the sorc casting chant
then again by the attacker if he has any +% fire dmg (also using his own -% fire while the sorc's fire pen doesn't matter unless she attacks herself)