Orcs – discussion about the race.

Against such a powerful ballista, I wouldn't hesitate to use the fury of the Stronghold or, perhaps, a single shot from the centaurs. And to reduce the danger of the Sylvan Elf's spells, I would try to eliminate the druids as quickly as possible. There's nothing you can do about a sworn enemy, of course. But you can increase your magic resistance and equip a few artifacts that boost spellcasting. I probably wouldn't invest in anti-chaos skills, as it's not uncommon for elves to use light magic.