How to beat 15 Pythium mages plus lions and wyverns

[Viajero]

Im currently sieging a Pythum castle where my opponent has around 15 Pythium mages and its God (a rainbow mage) but no meaningfull army, just a significant number of conjured troops (grand lions, wyverns, eagles, some of them magical etc). Some of his mages are level 4 in astral, and one of them is a 5.

It is around turn 33, and it is a MA small map no mods map so research is in the 2300's or thereabouts (i.e. magic levels 4 or 5 tops or possibly 6 if specialised).

I am fighting with Ermor and have around 5 or 6 arch theurgs and around 10 theurgs plus a bunch of undead, principes, shadow vestals, a few behemoths and banes etc etc (shall I put the behemoths and / or banes in front?). I am a noob, my opponent is a veteran so a bit scared here of possible surprises appart from the obvious ethereal casts on the conjured/magical troops, soul slay's, magic duels etc. Do I need magical weapons to hit magical creatures? what is the quickest way to get good numbers of them?

Given this is still early game in terms of magic, what would you think is the best way to assault such a castle?

Any advice appreciated,