Here are my fixes. Nice overview, but yes, there are some flaws that can be corrected by switching up sets.
Firstly, give Terrakion a Choice Scarf set. That's why Terrakion is even here: to get rid of dragons.
On Venusaur, I would use a Timid nature with a spread of 72 HP / 4 Def / 252 SAtk / 180 Spd. This outspeeds Scarf Lati@s, a real threat to your team.
On Ninetales, you really want a defensive set to survive as long as possible. I personally like the following, a SDeftales that can speedcreep everything.
Ninetales @ Leftovers
Trait: Drought
EVs: 248 HP / 212 SDef / 48 Spd
Calm Nature (+SDef, -Atk)
- Sunny Day
- Will-o-Wisp
- Flamethrower
- Roar / Pain Split
From Lavos Spawn's Simulation of a Drought team.
Yes, Donphan or Landorus-T should go over Charizard. Xatu should go over Forretress. Note, with the Donphan, I'm slashing Rapid Spin as a replaceable move because you may not need it if you use Xatu well. Feel free to use it as you want. Ice Shard I feel is a must to make up for the absence of Landorus-T should you not use it.
Donphan @ Leftovers
Trait: Sturdy
EVs: 248 HP / 80 Atk / 176 Def / 4 Spd
Impish Nature (+Def, -SAtk)
- Stealth Rock
- Earthquake
- Ice Shard
- Rapid Spin / Knock Off / Stone Edge
Or
Landorus-T @ Leftovers
Trait: Intimidate
EVs: 200 HP / 64 Atk / 244 Def
Adamant Nature (+Atk, -SAtk)
- Stone Edge
- U-turn
- Earthquake
- Stealth Rock
and
Xatu @ Leftovers
Trait: Magic Bounce
EVs: 248 HP / 204 Def / 56 Spd
Bold Nature (+Def, -Atk)
- Heat Wave
- U-turn
- Roost
- Toxic / Thunder Wave
~AoH