Is it a hot take to think that Grimmsnarl is a bit busted currently? Have played a decent # of games, but not too high up on the ladder yet, so take my opinion w/ a grain of salt, but the only viable counterplay to grimm that I've done so far is setup to +2 / +4 with Gholdengo / Kingambit / Chiyuu as grimm goes for Screens & abuse the momentum loss they have by switching girmm out by either taking out as many mons as possible or making it more difficult for them to setup w/ their setup sweepers or Cyclizar. Now, this has worked so far, but it feels like a fluke and something the Grimmsnarl player can easily adapt to. Landing a Defog is also another option, but grimm's team comps make it a bit harder to land that since they usually pack a Gholdengo or Anihilape that makes using Defog a bit risky.
An annoying element about Grimm compared to other possible screen setters is that OHKOing it is almost impossible, esp due to priority Screens, so its bound to get off both Screens in most cases. In one match I played, Specs Iron Valiant wasn't even guaranteed a 2HKO w/ Moonblast (tbf I landed an unfavorable range, but its still illustrates the point that the mon is bulky af).
For me the most evil team run Revival Blessing and Defiant Ape. HP does not matter on Grimmsnarl and being able to do it twice is crazy with Screens