I'm leery of absolute counters in a game without readily available damage types. It's not like it was before in WoM, where there was different flavored elemental damage in the low end of every spellbook.  For a large part of the game your practical damage options are going to be physical, and fire *if* you have a fire mage.   At some point you can build magic weapons, *if* you have crystals.  Otherwise, most non-fire elemental is currently end game, with high level spellbooks.

As long as that is the case, I think non-unique monsters should always be able to be "chipped away" at, as a last resort. It still makes those monsters very difficult to kill without the proper skills.