How to beat spider boss with soothsayer?

[Jaedar]

As Thread title.

I can't seem to do it. My troops die too quickly to retaliation. My offensive spells either do too little damage, or cost way too much mana to use for more then a few rounds. Summons are a no go as they all die really quickly and cost a lot of mana. Don't really have much in the way of buffs except for hells breath and mana spring.

What am I doing wrong?