Here s my team:
Venusaur (My Mega)
Bisharp (None 4 acrobatics)
Muk (Leftovers)
Gallade (Amulet Coin)
Snorlax (Leftovers)
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And here are the deltas in my box:
Sunflora, Clamperl, Haxorus, Budew, Geodude, Serperior, Liepard, Larvesta, Petilil (W/F), Scizor, Bergmite, Ambipom, Litwick, Misdreavus, Cyndaquil, Heatmor, Dunsparce & Solosis.
Which ones could I use for elite 4 and rest of story?
Because your team is slow, i think Delta Serperior would help. Otherwise, a Swords Dance + Priority (Aqua Jet is better, but Bullet Punch is a reasonable alternative) Haxorus would be great.
D. Larvesta and Hax. You may want to get rid of Muk for more offensive presence unless you really like him. D. Larvesta actually makes a solid tank with no weaknesses that can hit hard.
If you use D. Volc as a defensive mon, go special and make D. Snorlax your physical tank.
Bulky Volc @ Black Sludge/Volcarona Armor
Ability: Levitate
Nature: Timid/Calm(Timid/Modest for offensive)
252 HP, 100 Spe, 156 SpD(You can shift the SpD EVs to Speed but it doesn’t have QD so it kinda needs those)
-Roost
-Toxic Spikes/Tailwind(you could also run another coerage, but then you might as well just run full offensive, full offensive no investment Volc still has amazing SpD)
-Snarl/Dank Pulse
-Sludge Wave
The ideal set for Hax is
D. Hax @ Choice Band
Ability: Heavy Metal(If you run Heavy Slam) else run Strong Jaw or Water Veil if you’re worried about burns
Nature: Adamant
EVs: 100 HP, 252 Atk, 156 Spe
-Aqua Jet
-Heavy Slam/Iron Head
-Earthquake/Dragon Tail
-Bullet Punch/Draco Jet
Hope this helps!