This section largely exists based on my personal opinions. I am, by no means, an expert on Pokémon battling, especially competitively. I am mostly a solo player who occasionally participates in friendly battles with friends, so if you're looking for the absolute best strategies on competitive battling, you should probably look elsewhere. This section simply contains information on what works best for me personally.

To start with, I would look for a nature for my Bulbasaur that emphasizes it's high Special Attack stat. Bulbasaur's highest stats are it's Special Attack and Special Defense, but my particular playing style is largely dependent on decimating my opponent's health bars. I want to destroy them before they have chance to work on any fancy battle strategies. Therefore, I would want to get a Bulbasaur with a Modest nature, which raises Special Attack at the cost of Physical Attack. As much as I love Bulbasaur moreso than it's evolutions, it doesn't make sense statistically to stop it from evolving. (Though I have done so on a few playthroughs before.) Ivysaur and Venusaur share it's stat strengths, so this is a good nature for the three of them.

The next thing I would look for is my Bulbasaur's Ability. It's standard Ability is Overgrow, which increases the power of Grass-type moves when Bulbasaur's health is at 1/3. It also has the option of a Hidden Ability called Chlorophyll, which doubles Bulbasaur's speed in Sunny weather. Bulbasaur is a pretty slow mover, so this ability is handy if you make frequent use of weather-changing moves in your strategy. I, however, do not. I really on sheer power, so it's regular Overgrow Ability works perfectly for me. If you Mega-Evolve your Venusaur in combat, these abilities don't mean much anyway. Mega Venusaur's Ability will always change to Thick Fat, which halves damage from Fire and Ice-type moves, two of it's type weaknesses. This is a great Ability, and I wish I could have it in my Bulbasaur's non-Mega-Evolved forms, but it's not meant to be. :(

Now, since I'm mostly a semi-casual battler, I don't know very much about IV and EV training, even with the introduction of the Super Training addition to your Pokénav in Pokémon X, Y, Omega Ruby, and Alpha Sapphire. I do plan on looking into it more in the future, but I'll keep silent on it for now since I'm kind of an amateur. There are plenty of other resources right now online that are more informative than me on the subject.

Anyway, since I'm a power hitter in battle, my Venusaur's moveset would be something like: Solar Beam/Petal Dance, Sludge Bomb, Synthesis/Giga Drain, and Hyper Beam. I like my Venusaur to have maybe one or two heavy-hitting Grass moves, but since it's also a Poison-type, I like to give it a Poison-type Special Attack like Sludge Bomb. Synthesis or Giga Drain will help regain some HP if I don't take out my opponent quickly enough, though the latter will damage the opponent at the same time at the cost of potential healing power. Hyper Beam is handy if I'm fighting an opponent that doesn't have a Grass or Poison-type weakness. Normal-type moves deal regular damage to almost all types, excepting Rock and Steel, so it's a good, powerful backup move.