Fighting dragons tips

[bukimi]

Unfortunately I don't have summoning units to hire.
No demonologists on any island.
Thorns are no good for dragons

Is it profitable to cast summoning spell or is it better to cast damaging spells?

Knights have bonus against dragons. Are they really effective? I have resurrection spell or can accept losses.

Demons can be resurrected with resurrection spell, right? Maybe some executioners or demons will be also a good idea then?

I have access to 3 "call colossus" spells (1 already+2 in near shops) so can adjust my stack for these battles...