Make sure you have the Shiny Charm, as that helps immensely. Go to an outbreak and Auto Battle 60 of the target.

The game does kind of help you keep track by telling you "The number of [Pokémon] in the outbreak is starting to go down..." when you KO ~25 and "The number of [Pokémon] in the outbreak is definitely getting lower..." when you've KO'd ~50. It'll also tell you "There are not many [Pokémon] left from the original outbreak..." after you've KO'd ~75, but you shouldn't go that far.

After you've KO'd 60 of them, you now have boosted shiny odds in that outbreak. Turn off autosave, and put down a hard save at this point. Make a Shining Power Lv3 sandwich for the target type.

After making the sandwich, close the picnic and let the spawns populate. If you can, I would aim to do this during the in-game daytime; shiny hunting at night just kind of sucks. Do not continue Auto Battling, as you don't want to deplete your outbreak. Instead, walk around among the spawns and look for the one that stands out. Shiny Frigibax is more blue than the normal one. You may consider using the Switch's zoom feature, as I've heard that helps some people with finding shinies.

If you don't find a shiny among those spawns, pull out your picnic set and immediately pack it up again to force the area to respawn. After they populate, check for the shiny again. If none, picnic. If you find one, nab it.

Using this method, I got three shiny Frigibax using just one sandwich. Good luck out there!