Max whatever you need at the moment (either a Grass attacker, in which case go Roserade unless you are facing Ground-type attacks, or a Dragon attacker). If you don't need it at present, best to save until such a time where the dust use is warranted.
For getting more dust I'd suggest:
1) Go+ / Gotcha / Poke Ball Plus / whatever accessory: These are stardust making machines in a nutshell. As long as you keep a steady supply of Poke Balls, all you need to do is let those do the work and you get dust without thinking too much about it. It builds up quickly. Just watch your storage.
2) Fast Catch: This is a primordial technique (
link here for the unaware) for anyone who wants to take advantage of basically anything.
With Fast Catch, all you do is throw the ball and exit out. You can catch everything in your path with time to spare and that builds up dust.
3) Money extras: Incubators + Star Pieces. Incubators passively add to stardust count if you're continuously using them while Star Pieces obviously increase what you get w/ any action.
4) Any excess berries (e.g. Nanabs) you have can be used to feed gyms. It's minor (only 20 dust per berry) but it adds up if you're constantly disposing of them - this pairs well with the Gotcha in areas with tons of stops / gyms.
5) Stacking Quests: Stacking 2nd/3rd stages within your quest line-up allows you to later build up dust within a certain timeframe (E.g Double Dust Event). To stack, all you do is claim your quest reward and immediately run away - it is then added to the top of your quest list (the stack) and you can store up to 100 before it starts erasing them. The Equinox Event itself has an "Evolve 5 Grass-types" quest that awards Venusaur, so you can stack those.
Addendum to quests: Take advantage of quests that award dust, whether it's the generic ones available all year long or event-specific ones like Catch 10x Grass-type Pokémon -> 1000 Stardust. This also applies to quests that award 2nd/3rd stage Pokémon as well.
Aside from these, obviously take advantage of events where Double / Triple Stardust is promised by using all of the above.