Ah rock, you'd think it would be a good defensive type with it being hard and all that but IT'S NOT! It's weak to pretty much every type that matters, and in a mono-type run there's no using other pokemon. Still, through a lot of trial and error, I've assembled a team of experts that got all the to Eternatus and rolled over it like a boulder.
Oh, and the V-day event moved the DNA splicer from master ball tier to great ball tier, so the strategy was a bit different; I could actually count on them appearing.
Aeronflame is a fusion of Aerodactyl and Talonflame. Aerodactyl carried through the first few areas, and then I weakened it considerably by fusing it with a Fletchling. After it evolved, it was still weaker than a pure Aerodactyl, but at least it had Flame Body and Flare Blitz. Flare Blitz synergizes well with Orichalcum Pulse (Koraidon's signature ability that creates Sunny Day and inceases attack during Sunny Day), but I didn't use it very much; recoil from Flare Blitz is nasty, and I always use it sparingly. Then, after Aeronflame mega evolved, its Talonflame half started to hinder it even more. Still, it remained the MVP of this team due to its speed.
Nihilego was the token special attacker. Its Earth Power was essential early on when it was just it, Aerodactyl, and a useless Chewtle. Levitate removes Nihilego's greatest weakness. It served as a counter to rock and poison types, but not its fellow Nihilego's, no sir. Lusamine's Nihilego also had Levitate.
Dreadnaw began as a Chewtle that couldn't be used. Then it evolved into a Dreadnaw that wasn't worth using until it learned Liquidation. Well, Ice Fang was useful. Then, when I found a G-Max Mushroom, Dreadnaw turned into a monster that would tank any hit that wasn't grass type. For some reason Eternatus kept blasting him with Cross Poison, and that gave me a lot of time to deal damage.
Magcargo was a piece of semi-solid crap.
The unusual fusion of Shiftry and Garganacl,
Shinacl, wasn't used extensively either. I thought having a grass pokemon would be poggers, but I never found many opportunities to use it. I had it beat up Dewgongs in the ice caves, but turns out rock doesn't resist ice after all. I've used worse pokemon.
Now,
Steelith was a bit more useful than I anticipated. The Gigalith admixture gave it a little more strength, but in the end it mostly just lowered Mega Steelix's defense. Sand Stream didn't hurt, but didn't help much either. Frankly, without the mega form, Steelix wouldn't even be considered in a normal run.
Edit:
Bug run didn't take long, compared to the rock and dirt type. There were some catastrophically bad attempts, but I figured out that I should prioritize quantity over quality this time, and at the same time remember that I'd have to fight Eternatus in the end. So instead of taking Slither Wing or Pheromosa, I picked one mid-tier bug (Heracross), and a bunch of cheap crap with decent unlocks. Eternatus died at a record pace.
Weedle, being a quintessential early-game bug, might not have been an obvious choice, however its egg moves are amazing, its final form can mega evolve, and it's only 0.5 point.
Beedrill could hit even levitating steel-type pokemon with its Thousand Arrows, which happened exactly once. Toxic Spikes also went mostly unused. I used Beedrill caveman style, but it worked.
Vikavolt was caught just before my battle with champion Geeta. I spent a lot of money to teach it good moves, then ended up using it only once - deleting a shield on Ivy's Noctowl, not even defeating it.
Blipbug is another one of those shitty larvae, and just like Weedle it has the potential to become a being of incredible power.
Orbeetle could straight up erase enemies with Luster Purge, the drawback being that I had to buy ethers ever so often. That being said, Revelation Dance and Energy Ball also took down many pokemon. It's worth noting that Pokerogue changes how G-Max works, turning the forms into glorified megas.
Duradueye was a fusion of Durant and Decidueye. I think taking away Durant's steel type ruined it, and the increased special defense did nothing. As a Durant, it saw minimal use, but after fusing, I never used it again.
Herapom was the heavy lifter. Even as a Heracross it carried the run, but then I found an Aipom with the hidden ability Skill Link, which synergizes perfectly with Heracross' passive Technician. Yeah, it's Toucannon all over again except strong. I know Ivy's Mega Rayquaza would have been unbeatable without it.
Venodam was supposed to put Leech Seed on Eternatus as a part of the long term strategy, but Heracross and Orbeetle handled the space tapeworm before I remembered I had a strategy. Even so, this guy was useful in some places due to its wide array of special attacks. Fiery Dance would increase its special attack by two stages thanks to Simple.