Synchronize might be what you need.
Synchronize is an ability that 15 Pokemon can have: Abra, Kadabra, Alakazam, Mew, Natu, Xatu, Espeon, Umbreon, Ralts, Kirlia Gardevoir, Munna, Musharna, Elgyem and Beeheeyem. Chances are, you'll be able to catch one of these.
When a Pokemon with Synchronize leads the party, there's now a 50% chance that any Pokemon you encounter will have the same nature. Obviously, since some of those Pokemon listed above can be pretty easy to find, it'll be easier to get one of them with the desired nature. You can even save many Pokemon with different natures if you plan on getting something with a good nature often.
This way itll be much easier to get the nature you want, since you'll only need to soft-rest a few times to get one with the right nature, but keep in mind that IVs are unaffected, so you might need to soft-reset more for IVs.
Hope I helped!