It took me less than 30 minutes, actually, the room where Volcarona is located, and the room right before him, only Claydol will be encountered, and Larvesta spawn in there as well (obviously I was hoping someone would give me some super secret lowdown on the Larvesta situation, just casting the line out) but I definitely figured it out, those two rooms are better for Larvesta, because the one with the Staircase can give Cofragigus, Krokorok, and hordes, but the two Volcarona rooms only give Claydol.
I actually got two Larvesta in a row in those rooms, they're technically better, I swear by this
I was just hoping I would get some amazing answer like some random room in the Relic Castle would have like a doubled chance or something unheard of like that