Ok, the fireball is the problem then. So Lich Form is out, as it would decrease your resistances to magic damage as well as not being able to mount as you pointed out.
How is your Resist Magic skill? If not maxed, it could help decrease the damage received.
If your Resist Magic skill is maxed, an Arch Blessing spell could increase the skill more as well, and decrease the damage more as a result.
Are your pets max level and are they breeder pets or ones you tamed? 2nd and 3rd generation pets are usually more resilient to damage as well as being stronger. Normal or Bio pet?
Have you attempted the fight with more than you and your pets? Perhaps 1-2 more players would help spread the damage out a lot, amongst the other players and their pets.