Defeating the spider on impossible

[Vilk]

Yes there isn't always Royal Thorns, myself I got 4 in pirate isles and used sacrifice to increase the stack.

The double shaman stack is a nice alternative, three stacks with one of one shaman that you take care to protect could work too and be easier... or not. For the thorns that could be probably more hazardous if the last stack get killed. You could use gift to restore the summon talent and used thorns fighters and hunters.