... has anyone seen 42K HP Gremlin towers?

[ivra]

No!!!

I have however notice a big variation in how difficult the battles are. In my current game (Normal difficulty) I am using three "Ring of the Snake King" rings (Yeah, I know I am lucky! They each give +5 Intellect, which is perfect for my blaster-mage). I have regularly subdued them and I notice that the health of the towers is always the same for the same ring, but the health varies a lot. For my three rings the health of the towers are approximately 2000, 3000, and 8000. That is a variance of 4x between the weakest and the strongest ring.

I loaded my normal game and the health of the towers in the hammer battle was 11345, which will correspond to a health of approximately 19300 on Impossible. If we assume that this is a middle value and multiply it with 2 then it is getting close to the value you got.