[Mightypeon]

I kinda agree on Zmeys being a bit too tough, too cheap (since they can be totally decent with 15 gems of gear, just about anything in their weight class would need more) and available too early.
I actually like giving them cold blooded as a glaring weakness.
They are very hard to kill, even compared with actuall high end summons costing twice the gems.
Grendels are blobs of hitpoints with full slots, that niche is actually suprisingly empty (Iron Angel?) apart from them. Elemental royalty is indeed a bit underpowered in general, but comparing them with Grendels is imho not a good way to establish this.
I have no hard opinion about Shishis and Krakens.
I find Wendigos interesting, while they are available quite early, it will take between 5-10 turns for them to be battle capable, and thats actually quite interesting.