I came to mostly the same conclusions
, except I prefer Emerald Whelpling to Sprite Darter Hatchling: heals and shields are much more potent when incoming damage is nerfed, and Sprite Darter Hatchling doesn't get to go first against a Flyer so Evanescence is only a 1 turn dodge instead of the usual 2.
I also can't find a Flyer with a mechanical attack, so anti-Beast is the least obvious choice for me: I like Darkmoon Zeppelin for the decoy ability and a choice of burst damage abilities (Explode is 900+ damage against a Beast). I've also been happy with various Moths, and Pterrordex Hatchling is very good with high power and a solid heal.
In practice, a crab or Magical Crawdad is very strong against critters, since they tend to have flurry or stampede as their big damage moves, and Shell Shield subtracts from each hit of those attacks. For the same reason, Jade Owl is rather lackluster against crabs, since his flying attacks are multi-hit and get heavily nerfed by Shell Shield, and the crab is typically doing damage with a Beast attack instead of Aquatic.