Venusaur is one of the best grass type to use to abuse sun, it has good setup move in Growth, good offensive stats, good speed, and also good coverage moves. Use Solar Beam as your main grass stab if you want damage and use Giga Drain if you prefer recovery. Sludge Bomb is a good reliable poison stab to use but feel free to swap for other coverage if needed. For the third slot you can choose between Earth Power or Weather Ball depends on the matchup, both are great. And your main setup move is Growth as it raises both of your offensive stats by 2 stage if used under sun. Lastly you could swap out your held item anytime too but generally Life Orb is very good, another alternative is Focus Sash to guaranteed a setup or living a hit.
