I love this idea, and might just have to steal it !
I reckon you could make a whole adventure out of it; a seemingly endless series of iterative upgrades, each one nerfing a specific advantage of the party. Mage doing too many buffs ? The construct's weapons apply a Dispel Magic to a character when they hit. Invisible characters ? The constructs puff out clouds of glitter each round. And so forth, one upgrade at a time until the party start to feel doomed....
100%! There are lots of great upgrades that could be applied but they wouldn't be targeting the specific builds of this party.
I am still trying to work out a great way to counter the druid summoning bears but the dispel magic attack isn't a bad idea. Maybe like a smoke grenade or a dart or something that does that.
The homebrewer in me thinks that a "Create Picnic" spell should deal with the bears nicely 😁