on easy and normal dif both kraken and spidy where no real issue for any combination of army I had.
But on Hard and Impossible it started to be an issue.
Both spidy or kraken fights can cause a lot of trouble as even if the boss is defeated, still the hatchlings are there (when the combat takes too long, it starts to be like real problem : in Kraken 2x 500 devilfish and raising each next turn, with spidy even more spiders are summoned, and not only 2 stacks but 3 or four).
As I didn't want to wait till my firepower (leadership mostly) is on much higher level, I beat both kraken and spidy with undead army (problem is that only ghosts and vamps survived... where the funny thing is that gosts even if not controlable later on, survived with + 100% ~ 500% more than at the beginning.
Vamps on the other hand are great as have no retaliation - so they can attack the boss w/o any reaction.
If undead are not the choice for you, Kraken can be beated by bandits and snakes (both have distant attack - which does not provoke retaliation, in the other rounds they can clear devilfishes)
also it is good to have one unit stack that is not of high importance to you but has enough health (casting stone skin or other defence spells helps too) let this unit stay near the boss (spidy or kraken) so the boss will attak instead of summon... by this you can protect your archers much better.
For the spidy are snakes and bandits not the best units (are far to weak, well, the blue snake kings are not weak, but I mostly ever had better units already) so here I would advise to build (if bandits nor undead is a solution for you) army of archers and duplicate them with CLONE spell, defending them with one strong army (like giants?)