Getting past the bloody spider.

[DeepSoul]

Well, I have Ice Dragons too but they can't be healed by that spell unfortunately since they are immune to magic, even if it's an ice creation(a mistake by the devs?). You can have one minion at a time but you can heal it, and they get a 50% attack penalty in caves, so it's not as good as you think but still worth it because they can tank the spiders really well.

I managed to beat the boss as a level 17 mage on hard and also got achievement. Here's what I had:

1 Black Dragon
2 Ice Dragons
3 Trolls
15 Jarls
80 Royal Snakes

Items: Healer's Jacket (20% poison resistance). Bought from pirate captain on Isterreng.
Spells: Ice Snake(cast on boss), Magic Spring(to get more mana for Ice Snake)

The Dragons tanked the minions while the rest attacked the boss whenever it was possible. I used the Jarls ability to give another turn to the snakes and they did a few really high hits on the boss. They even managed to use lunge without getting hit a few times, but don't attack his head because then he will use his magic.