Ninetales @ Leftovers
EVs: 252 Speed, 252 Sp. Atk, 4 HP
Sunny Day/ Calm Mind
Will-O-Wisp for Attack drop (so it won't take that much damage from Waterfall), Clam Mind for SP. def boost, or Sunny day if you want to counter a Politoed switch in.
Skarmory @ Leftovers
EVs: 252 HP, 252 Def, 4 Spd
You already have a spiker in the form of Forrestress. Also, Skarmory is susceptible to Taunt with your current set. You can choose to make Skarmory specially defensive if you want, since it already has a base def of 140.
EVs: 252 HP, 252 Sp. Def, 4 Def
Volt Switch/ Gyro Ball
I don't see much point in counter; most of the moves Forrestress will be facing are special moves like Flamethrower and Surf, since most people know that it's a physical wall. Either go with Volt Switch to deal some damage when switching out, or Gyro Ball for max speed difference damage.
Dragonite @ Leftovers/ Lum Berry
EVs: 252 Atk, 252 Speed
Outrage/ Dragon Claw
If you're putting Dragon Dance on your Dragonite, there is no point in putting Dragon Tail on it. If you want it to be a para shifter, invest EVs in its defences/ HP and take out Dragon Dance. There is no merit in Dragon Dance/ Dragon Tail, since you will be going last all the time, which gives your opponent an opportunity to Ice Beam you. Extremespeed gives no STAB, so your opponent will be able to take the hit (on something like Porygon2 with Eviolite).
Sawsbuck @ Life Orb
EVs: 252 Speed, 252 Atk, 4HP
Your main Sun rival is Venusaur (Jumpluff will sleep powder you, but it won't sludge bomb you); at 252 EVs Sawsbuck will outspeed it. Jump Kick is resisted and has a crash recoil, so best go with Return for STAB. Nature Power is there to deal with Fire Types.
Imo, Gyarados doesn't fit well on this team... in Sun Waterfall will only be half as powerful even with Dragon Dance (if you manage to set it up), and against a Rain Team it will have little advantage, especially since many Rain teams carry past Thunder sweepers like Jolteon, which will outspeed it even after 1 dragon dance. Against a Sand Team, Gyarados will manage against Hippowdon, but Tyranitar will be able to Stone Edge it.
If you need a rain and sand counter, go with Gastroon and Storm Drain. It will not get buffeted by sand, take no damage from water attacks, and if it faces a grass type, you already have other Pokemon to deal with them.
Gastrodon @ Leftovers
Ability: Storm Drain
EVs: 252 HP, 4 SpA, 252 Sp. Def
Basic Gastrodon set; specially defensive tank.
Also, I would replace Skarmory for something else, maybe Venusaur, and put Stealth Rock on Forretress instead. You don't need two entry hazard set ups.