Venusaur's main issue is above; but this is the set I tend to run:
Venusaur (M) @Focus Sash
4 Atk, 252 Sp. Atk, 252 Speed
Trait: Chlorophyll
Hasty Nature (+Speed, -Def)
-Growth
-Giga Drain
-Weather Ball (150 Base Power Fire attack in Sun!)
-Earthquake/HP Rock
Also.
- Espeon would work over Xatu as a magic bouncer, with more offensive capability and supporting moves.
- Raikou with Calm Mind/Thunderbolt/Weather Ball/Aura Sphere would work as a much better Rain counter, taking pittance from Water moves, and returning fire with a huge Thunderbolt, plus it can kill Tyranitar with Aura Sphere, and can take out a lot of things in sun.
Overall rate is a 7.5/10, nice work.