I still don't know of a proper counter to Merlin spam.
The only all-terrain assault is the Hermit and that has too little HP to attack a ball (AoE is a killer), along with being too slow. Knights are far too slow.

Maybe a big Thug+Aspis ball (though that has fairly cheap counters in the form of cloaked snitches, charonned snitches, cloak-lobbed-snitches and skirmishers)?
Nah, just tried that in sandbox. You'd need to go all-in with a force that is only slightly smaller than the Merlin ball.

I guess in theory Tremors+heavy shields would work, but that's very specialized so it will get eaten by everything except a Merlin ball, takes quite a while to work (giving the enemy time to retreat, replan or counter) and doesn't work at all until it suddenly does (so you won't be doing any attrition damage and there's no point in a half-done Tremor+shield ball).