There are 2 ways to obtain a shiny Cacnea in the Asado Desert, these are of course excluding finding one randomly, like I happened to do (you can also get a shiny Cacnea with the Masuda Method but nobody's about to sit here hatching a bunch of eggs).
1: Make that sandwich and run around, going against the odds like the absolute MAN YOU ARE!!!
There may be some Bramblin (depending on which side of the map you are on) and a couple Capsakid but pfft, that's nothing. I should also mention Cacnea only spawns on the East side of the Watchtower, which doesn't really matter when you use
2: Resetting for a Mass Outbreak (the more efficient way).
For those who don't know, making an Encounter Power Sandwich (at least I think it's encounter) boosts the odds that when you reset your outbreaks, you will get more outbreaks of that specific type. Unfortunately, outbreaks no longer take into account the player's location, but we can still reset for Cacnea outbreaks, although it may take a while.
To do this, we're going to make a Grass Encounter sandwich (sparkling power and stuff too, we'll get to that in a bit), open up the map, zoom out to the second to last smallest dot by pressing ZL once (don't close the map until you get the outbreak), and then reset your outbreaks using the Date and Time method. This only takes a few seconds of in game time, but once you reset your outbreaks, you're going to want to wait a few seconds to make sure all the outbreaks pop up on the map before repeating the resetting process again if you didn't get the Cacnea outbreak. This shouldn't take too long, and once you get the Cacnea outbreak, use the rest of your sandwich to actually hunt it.
Sammiches!!!
I usually get my sparkling power recipes from this site, and I use the pickle and cucumber recipes, but I'll share both recipes here:
Recipe 1
> ・Lettuce
・Salty Herba Mystica
・Sour Herba Mystica
Recipe 2
・Cucumber
・Pickle
・Lettuce x3
・Any 2 Herba Mystica except Sweet + Sour
These should get you Encounter Power 3, Title Power 3, and Sparkling Power 3. And of course, if there are other recipes you happen to find online or whatnot, feel free to use those too.
Sorry this was so lengthy but I hoped this helped. And before you say "But Fish, sir, I'm not a man," we're ALL men, we just haven't realized it yet.
That was a joke, guys, don't get me cancelled on Blue Bird or Zed or whatever you call your social media apps these days...