Hmm, if you want the boss levels to be easier simply play on impossible difficulty
Seriously, the main quests are incredibly easy on this difficulty level, i have more problems with random walking monters on the map
I beat the spider boss without taking any damage whatsoever. I was lvl 17 at the time, my army composition - Canoneers, inquisitors, evil eyes, cavaliers and royal snakes. This is one of the only battles where i actually used fits of energy or however the spell is called but it wasn't necessary
The stacks don't grow but instead depend on the spider's healts - from 50k to 33.333 health the spider throws 130-240 spiders to reach something like 250-400 for when the health is below 10k and the spider teleports for the last time. Anyway almost all the units were able to inflict enough damage to kill the little spiders from the first shot and after that i used the remaining troops to inflict damage on the spider queen