(For Either Forms)
Timid Nature Thundurus @ Zap Plate
252 Sp. Atk, 252 Spe, 4 Sp. Def.
-To Be Used In The Rain-
- Focus Blast
- Flash Cannon
- Sludge Wave
Focus Blast - With Focus Blast in play you automatically have both weaknesses (Ice and Rock) out of the way. Since it does hefty damage already, it is sure to pin the enemy with the provided Timid Nature (+Spe, -Att). The Timid Nature is one of the reasons I didn't use an Attack (Stat wise) Thundurus. This move also has potential to lower Special Defense.
Flash Cannon - Flash Cannon is a cool move to come out of Therian Thundurus, but that's besides the point. It's a Special Attacking move so that's a +1 with my nature, but the cool thing is is that it takes out both of Thundurus's weakness types as well. It's 100 accuracy so that'll be really efficient, and it takes care of dual typings that are a strong suit against you that normally wouldn't have been super effective without the use of Flash Cannon. Examples of Pokemon with those weird dual typings: Articuno, Alolan Ninetales, Carbink, Diancie, etc.
Sludge Wave - Sludge Wave is a better Sludge Bomb and is good to have just in case. It is the faulty move that was extra and provides coverage to Pokemon that can't be penetrated by the other moves. This list includes most grass types. So, in a way, this is replacement for Acrobatics or Fly.
Thunder - This is something truly fun to work with. Thunder starts out with 110 power. Now, you add STAB which makes it a 165. Then you apply the Zap Plate which does 20% more damage to all Electric type moves which makes it 198. If you use this in the rain, it's kind of OP how it tends to not miss a lot (almost caught myself saying "ever"). If you get a beneficial nature to Special Attack, I'm sure you can master this moveset.