Well, there are a few things. First, Stealth Rock prevents Darmanitan from switching in and out like crazy. It also wears it down. So keeping up rocks can be a good countrplay, especially if you can play around the choice lock.
Max health Machamp can usually live an attack and 1 shot return it with Dynamic Punch or a similar fighting move. If you are adding Dynamax to the equation, Dynamax Machamp will also 1 shot Dynamax Darm. I have also been experimenting using a semi-Trick Room team with Machamp and Copperajah, which results in a very clean 1HKO for either if you can get the Trick Room up first.
Scarfed Imposter Ditto can generally also beat it because it can get the Flare Blitz off first. Although, most of the Darm sets I see are Choice Scarfed. Ditto or Galarian Darmanitan can 1HKO each other with the support of Sticky Webs.
I assume a high powered Bullet Punch abuser like Lucario (with Stealth Rock support) or maybe Guts Conkeldurr with Mach Punch could one shot it. Same with a Choice Scarfed or Dynamax speed boost with Cinderace, Rapidash, or Charizard. I haven't actually tried any of those things yet myself. They are just estimations based off Darmanitan's frailty and maybe speed tier.
Defensively, most water types would resist both Ice STAB and Flare Blitz. So finding a good defensive water type could work. The main problem with Toxapex is it gets wrecked by Earthquake coverage on the wrong predict. I think Intimidate Gyarados could actually be made to counter it if it wasn't already such a good Dynamax abuser. Although, Darm can carry Stone Edge if that becomes too common. Fighting and ground are the most common coverage moves carried in addition to the main fire and ice type moves. However, it should be noted that both Stone Edge/Rock Slide and Zen Headbutt are possible on it if certain threats and/or counterplays become more common.
