Toxic-Venusaur is nothing without this move
leech seed-restores health goes well with toxic
energy ball/sludge bomb-Goes well with higher spATK. I wouldn't use petal dance since it locks him into that move, allowing your opponent to switch into a different pokemon, one more resistant and ready to deal with your confused venusaur. sludge bomb has higher power, but isn't as useful since your opponent would be poisoned by toxic.
ingrain/ synthesis-ingrain if you want even more health at the cost of switching, synthesis for instant healing
it also works well to give him the item leftovers for more healing,
or you can make him a sub seeder:
last, you could take the unexpected route by using swords dance ti power up venusaur, then using physical moves like double edge and earthquake to take out your foes, while healing with leech seed. The only downside to this is his low speed. I've seen this before, it can be deadly when unexpected, many foes use taunt on venusaur, not knowing that it plans on attacking them directly. He has sleep powder to, letting you stop your foes while you power up.