Here's your Thundurus putting in some work in my first match of the season: JNHW-WWWW-WWW8-YSSA. Meowstic reduced to Struggle is a beautiful thing.
I ended up going from Thunder/Rain Dance to Nasty Plot/Thunderbolt/Hidden Power/Taunt because -- did you know this? -- it has Hidden Power Ice. Holy god, that's useful.
Rain also ended up being less important than expected, and it needed another attacking move. I've dropped Durant for the time being; wasn't as useful as I expected. It did net me some wins (finished in the 1600s last season but probably could have been higher) but sticking them with Truant Pokemon also meant I had a Truant Pokemon on the field, and trying to stick them with a second one could be a multi-turn guessing game. Overall it's just been much more comfortable to have at least one attacker out from the first turn.
What I replaced it with is fun, though: Lv. 1 Aron with Endeavor and Shell Bell. I don't normally bring it, but if Klefki isn't going to be much use (say a heavily ground team), I will. It can let Cloyster pop things off with Ice Shard or, more often, just give me extra turns to set up with Shell Smash since the opponent freaks out about Aron being around and is likely to go for Leech Seed, Will-o-Wisp, etc., which Cloyster laughs off with Lum Berry. Even when I don't bring it, seeing it in team preview usually means they'll bring a Leech Seeder/Sleep/Burner in their first three Pokemon, which also net free turns for Shell Smash, Substitute or screens.