Small demolishers can stay the same. But medium demolishers would be highly resistant to physical and explosive damage, but less to electric. They should however still be unkillable with Nauvis levels of electric damage + destroyer bots, because the idea is to lock the ability to kill medium demolishers behind Fulgora tech, specifically the electric damage upgrades. My original idea would have also left them vulnerable to explosive upgrades from Gleba, loosening the requirement to having beaten any other planet rather than Fulgora specifically, and also broadening the amount of tactics that could be used. But I realized that nukes and reactor explosions make that impossible in practice.
Big demolishers would essentially gain the stat structure of huge asteroids: Impervious to everything but railguns. However railguns still shouldn’t instakill them, it should still be a real fight that demands some attention and makes it worth it to invest into railgun upgrades. Frankly I believe the ridiculous interaction between demolishers and railguns, as well as the relative worthlessness of railgun infinite technology past the very first few levels, to be serious flaws of the way railguns work. But that’s a topic for another thread… viewtopic.php?p=653496&

Now, if any of you are concerned about the possibility of a softlock: Remember that it’s still entirely possible to mine in a demolisher’s territory while it’s still alive! It’s just tricky. Imo it’s just the correct level of harsh to players who delayed progression for too long. Also sulfuric acid, the planet’s fuel for power, is a pumpjack resource. So it will never truly run out, and you would at least be able to keep your roboports on indefinitely.
I also didn’t mention poison damage. I don’t have too much experience with using poison capsules, but my impression is that they can only be a supplementary weapon to use in conjunction with others. And they should be able to safely remain in that role without affecting the core purpose of the suggestion, and it just would add a bit of variety. But if I’m wrong and it’s somehow possible to kill a medium demolisher with poison alone, feel free to tell me

As a related side note, I believe Vuclanus’s starter patches should be dramatically shrunken down, and the first patches with any real long term usability should be in small demolisher territories. It would make players interact with demolishers more, besides the single obligatory one that’s guarding the tungsten. Besides, it’s shocking how much larger and richer Vulcanus’s starter patches are compared to Nauvis’s, when there’s no reason for them to be any different. If anything, Vulcanus should have smaller ones. It’s a later and by extension harder planet, and the player also has efficiency boosting technologies by the time they arrive to it.
But hey, I think this suggestion would also have the beneficial effect of making tesla weapons more important and attractive to research, as currently you can comfortably beat the game without touching them (I would know, because I’ve done so myself).