Mega Venusaur (Poison) for S Rank
Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 252 HP / 252 Def / 4 SpD
Relaxed / Bold Nature (+Def -Spe) & (+Def -Atk)
- Synthesis / Leech Seed
- Giga Drain
- Hidden Power Fire / Earthquake
- Sludge Bomb / Knock Off / Maybe both HP Fire and EQ
Synthesis is for instant recovery (25% in Sand + 8PP), Leech Seed is for continuous healing in small amounts.
EQ still 2HKOs Heatran with a negative nature.
Hidden Power Fire is slashed first because Nidoking can still be killed by Bullet Punch spam from Scizor whereas Nidoking can 1HKO Heatran with Earth Power.
Giga Drain is standard for Ground + Rock Monotyes
Sludge Bomb is for fairies, but you should have Poison STAB on at least 1 other Pokemon. Knock Off is for hitting Psychics on the switch, and having both would be the safest vs Steel Monotypes.
Explanation: Venusaur is already a monster on its own with its scary 80/123/120 defensive stats. It takes neutral damage from Fire and Ground moves (Which Poison monos hate), and it has Skuntank and Drapion to take Phychic moves for it who can possibly Pursuit-trap some Psychic mons that would threaten it. This time, its Grass typing is helpful since it gets STAB Giga Drain for Ground + Rock Monotypes (they resist Poison STAB). Venusaur also gets a lot of options. It can choose from a Leech Seeder, All Out Attacker, Defensive, or even a Curse set making it extremely hard to predict what set it would be running.

Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 252 HP / 252 Def / 4 SpD
Relaxed / Bold Nature (+Def -Spe) & (+Def -Atk)
- Synthesis / Leech Seed
- Giga Drain
- Hidden Power Fire / Earthquake
- Sludge Bomb / Knock Off / Maybe both HP Fire and EQ
Synthesis is for instant recovery (25% in Sand + 8PP), Leech Seed is for continuous healing in small amounts.
EQ still 2HKOs Heatran with a negative nature.
Hidden Power Fire is slashed first because Nidoking can still be killed by Bullet Punch spam from Scizor whereas Nidoking can 1HKO Heatran with Earth Power.
Giga Drain is standard for Ground + Rock Monotyes
Sludge Bomb is for fairies, but you should have Poison STAB on at least 1 other Pokemon. Knock Off is for hitting Psychics on the switch, and having both would be the safest vs Steel Monotypes.
Explanation: Venusaur is already a monster on its own with its scary 80/123/120 defensive stats. It takes neutral damage from Fire and Ground moves (Which Poison monos hate), and it has Skuntank and Drapion to take Phychic moves for it who can possibly Pursuit-trap some Psychic mons that would threaten it. This time, its Grass typing is helpful since it gets STAB Giga Drain for Ground + Rock Monotypes (they resist Poison STAB). Venusaur also gets a lot of options. It can choose from a Leech Seeder, All Out Attacker, Defensive, or even a Curse set making it extremely hard to predict what set it would be running.