>>52772458First of all, you want a wooper outbreak since it's literally impossible otherwise. To force outbreaks to change you have to turn the switch clock 1 min behind every time, and a good way to increase (but not guarantee) the odds of a wooper outbreak to spawn is to eat a sandwich of the pokemon's type and to do the time change on a habitat it spawns (i went with ground and the swamp since it has less overlap)
You won't necessarily get the outbreak on the area you're doing this but you'll get it somewhere relevant. Try to avoid areas with too much water or that have very spread out outbreak spawns since they're a nightmare to work with.
After that, just do the old KO 60 outbreak pokemon, save, and then make a sparkling + humongo sandwich (see picrel) and pray for the best. Good luck soldier, it might take several attempts for a max size one (you can soft reset every time the sandwich is unsuccessful or the size isn't what you wanted to save on herba mystica; remember to turn auto save off so you don't lose the ingredients and to be able to savescum in front of the shiny in case the love ball fails)
I went with love ball for mine and I love her, she's not max size but she's right under the 255 size so it's good enough for me)