Defeating the spider on impossible

[Metathron]

I just gave up in about round 20, cause those spider stacks kept piling up in greater numbers, to the point where I couldn't afford to attack the giant spider anymore, but had to focus all my units on the summoned spider stacks. By round 20, the giant spider was left with some 38,000 hit points (of 50,000). Jeeez!

I have shamans, royal snakes, giants, cannoneers and inquisitors.

Any tips?