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You need Synthesis, OP.
Giga Drain is great for recovery, Azumarill, Tyranitar, ect. The healing can help it beat certain bulky mons.
Sludge Bomb is good for Grass resists, such as Latios, Charizard, ect. The 30% poison chance is also clutch, and stacks with Leech Seed.
Leech Seed is another form of recovery that can deal residual damage to switchins, and help it beat Chansey.
Sleep Powder is useful, but can be restrictive. Venusaur needs all the moveslots it can get, and wasting one on a one-time sleep move cuts down on utility.
HP Fire for Ferrothorn and Scizor.
Earthquake for Heatran.
Synthesis is REQUIRED for a defensive set. Mega Venusaur's thing is being unkillable without a strong Psychic of Flying move. It needs reliable recovery to do this.