I like the idea of this set.
Gives it a lot of power, but gets nullified if rain isn't active.
Let me dumb it down for everyone how this set should be run.
Dragonite (M/F) @Yache Berry
EV Spread: 252 Sp. Atk/ 252 Sp.Def/ 4 Hp/Def
-Focus Blast/Dragon Pulse/ Hidden Power ( Fighting) /Roost
Rain Dance will NOT work as a last move for Dragonite, as it has to be the one to set it up. It's too slow to set up any weather, or Hazards. It's best to run this set with a Pokemon that has the Drizzle ability, or another pokemon that can set it up.
The set is simple though. Surf is basically STAB in rain
Thunder gets 100% accuracy, as does Hurricane, and Hurricane is STAB
Roost for Self-Healing, and keeping Multiscale ongoing, Focus Blast, or HP Fighting to eliminate threats like Ferrothorn, or go with the STAB Dragon Pulse.
As for the EV spread, it needs to be able to take hits from Sp. Attackers, mainly from Ice Types.
Yache Berry is to provide some resistance, as will be Roost, if you happen to move faster than your opponent. Multiscale helps with resistance even further, as it will take less damage while on full HP.