So, you just finished Dragon Ruins?
Yes I have
Ok. Normal difficulty?
Once again yes
Alright. So, the challenges I see ahead are Infernal Cult, Rain Gym, Persephone, Adam, Jaern, The Forgotten one and finally the E4.
And you center your team around Delta Venusaur, right?
Yeah
Ok. Delta Venusaur must be your mega as he is difficult justify using him otherwise. I got two sets for him: Offensive and Bulky Sweeper. Both looks similar, but the main difference is that the Offensive variant is faster and hits harder while the Bulky is slower but is more resistant.
Offensive:
Delta Venusaur @ D.Venusaurite
Ability: Psycho Call
EVs: 4 Hp / 252 SpA / 252 Spe
Timid Nature
-Moonblast
-Psychic / Psycho Boost
-Shadow Ball
-Calm Mind
Bulky:
Venusaur Delta @ D.Venusaurite
Ability: Psycho Call
EVs: 252 HP / 152 SpA / 104 Spe
Modest Nature
-Calm Mind
-Psychic
-Moonblast
-Shadow Ball/Leech Seed
His EV spread allow hit to outspeed 252 Speed Adamant Tyranitar.
You can change your spread with the EV reseter and the EV trainer in your secret base. If there are terms you don’t understand, please tell me.
I do understand
Next, even if it is just for The Forgotten One, train your Delta Gallade. The reason is simple: 5 of its Pokémon are either weak to Ice or Electric. The only real menace will be Mega Kangaskan. WATCH OUT: IT WILL BE AN INVERSED BATTLE. All weaknesses will become resistances and vice versa.
Otherwise, when holding Choice Scarf, D.Gallade can outspeed and Revenge Kill almost any Dragon and Water. This is important when you consider the Champion’s team (#NoSpoil). Let’s just say he has 2 dangerous Dragon Types. The Elite Four is have difficulty with Ice types in general
I personally used D.Gardevoir and D.Gallade in every Playthrough of Insurgence I made. He is not a mandatory teammate, but he is very useful against The Forgotten One. Your choice to include it or not.
Delta Gallade @ Choice Scarf / Never Melting Ice.
Ability: Volt Absorb
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
-Ice Shards
-Wild Charge / Thunder Punch
-Icicle Crash
-Earthquake / Custom Move [Ghost] / Swords Dance
Custom Move [Ghost] is here to hit Mega Kangaskan on it’s weakness. Lookout, Delta Gallade is frail on the physical side and without Choice Scarf and Jolly Nature, Mega Kangaskan has a good chance to outspeed.
Next, we have the Infernal Cult and Zenith. All of his Pokémon are either weak to Ground or Water. That mean Swampert is a solid choice. Moreover, he resist Steel and Poison, covering Venusaur’s weakness.
Here, use a physical attacker set with Waterfall, Earthquakes, Ice Punch and Portect. Make him hold Leftervers.
Protect is used to make pass Weather or Trick Room turns while gaining extra Leftovers recovery. The other moves are obvious, Waterfall and Earthquake for STAB and Ice Punch against Grass and Dragon.
Then, one of the most powerful Pokémon in your box, fits all the requirements to be a legends but funny enough isn’t a legendary: Volcarona.
Volcarona is among the few Pokémon I can 
myself on when I see it using a boosting move (Quiver Dance in his case). He is THAT strong. Also, Volcarona is a fast where Venusaur is a slower one. Volcarona also compliment greatly Swampert by destroying any Grass Type while Swampert take care of Ground, Rock and opposing fire. Finally, he destroy Adam and his Dark Type gym.
Volcarona
Item: Life Orb
Ability: Flame Body
EV: 252 SpA / 4 SpD / 252 Spe
Timid Nature
-Flamethrower/Fire Blast
-Bug Buzz
-Giga Drain
-Quiver Dance
Easy. Setup and watch your helpless opponent cry for mercy.
I would bring a Bulk Up/Belly Drum D. Snorlax, to complete the FWG core.
Adamant, ask @PeterHolmes74 for the spread. (If you run Bulk up, I prefer max SpD investment for a mixed tank)
- Bulk Up/Belly Drum
- Seed Bomb
- Synthesis
- Drain Punch/EQ
You could finish the team with Haxorus or Hydreigon, and possibly replacing Swapert or Gallade with D. Haxorus to complete the FSD core.
The true best Haxorus set
Adamant/Jolly Max Atk Max Spe Choice Band Struggle
If you use Bulk Up, use this spread:
Delta Snorlax @ Leftovers
Ability: Thick Fat
EVs: 188 HP / 144 Def / 176 SpD
Careful Nature
-Bulk Up
-Seed Bomb
-Earthquake / Drain Punch
-Synthesis
This is the standard Curselax spread. It makes him as bulky as possible. It usually setup slowly in the enemy’s face and get multiple boost, but Adamant and some EV is very viable. RestTalk is viable, but looses coverage. I personally don’t like Belly Drum because of how frail he becomes at 1/2 HP.
At 1/2 Hp, he can live a hit and synthesis and net KOs with Drain Punch/EQ, which I use more than Seed Bomb.