There's two great ways to farm sparks: Monstrosity and Abyssea. The former works for pretty much anyone, the latter is better if you have a job with spammable AOE. I went into Abyssea on GEO, stacked up magic boosting atmas and trained up colibri and spiders and -ra nuked the crap out of them with the "deal 500 damage 200 times" objective- you can easily complete that in a couple pulls since every monster hit counts. You can use your geomancy to avoid more damage and boost your nukes. BLU has been AoE killing stuff like that since abyssea existed (of course BLU has been doing this a lot longer)