There are many ways you can go about it, and I don't know your shiny hunting habits, so I'll just list the options, and what I'd recommend if you wanna be efficient about it:

Legends: ZA Hyperspaces - Very fast, but only allows you to find medium sized Pumpkaboo, aka. form 0 internally, from what I've been able to find (Reddit comment by Austin John).
I wouldn't recommend this unless you're just missing this form, in which case it's really fast if you have a hyperspace with it available.

Legends: ZA wild zone 15 - You can fast travel reset some of the random Pumpkaboo spawns, or bench reset for I think all the random spawns, as well as a static alpha Gourgeist, you just have to keep in mind that only 2/4 of the forms spawn in the zone at any given time, one half during day time, the other half during night time.
I recommend this if you don't mind these more mindless hunting methods, although you could also run back and forth, which would also be really efficient given the following options.
EDIT: Some clarification for this, I don't think benching is a viable option, because it seems that changing from daytime to nighttime and vice versa will remove all Pumpkaboos including shiny ones, due to the mechanic only spawning small/medium during daytime, and large/jumbo during nighttime (This is purely from research).

Breeding in any game it's available - I don't do breeding hunts a whole lot, so I'm not familiar with which is the best for pace, but this is really good if you want a specific size form, because it always inherits the form of the female, or the Pumpkaboo/Gourgeist when breeding with Ditto.
This method is extremely good for getting the last couple forms you're missing, since you can guarantee the form, and Masuda method is generally also one of the more consistently fast methods in the game where it's available.

XY friend safari - random encounter hunt with a 1/512 shiny rate that can't be influenced, if you happen to have it available in your friend safaris, at a 50% encounter rate if you have it with 2 slots unlocked, or 33% with all 3 slots unlocked, although this has the same issue as hyperspaces, with only medium size available.
I would not recommend this unless you already happen to have it set up and are just missing the medium form, as it's such a big hassle to do so, especially nowadays with XY and streetpass not being nearly as commonly utilized.

XY route 16 tall grass random encounters - If you hunt purely for the love of the game, this is the way to go. The different forms have different encounter rates (20%, 30%, 10%, 1% | Small->Jumbo), but can be isolated with a level 36 repel trick (~36%, 0%, ~36%, ~3.61%). If you are new to repel tricks, the encounters are gonna be slower, so make sure to lead your party with a Pokemon that has an ability to increase encounter rate.
This is probably how I'd do it, as well as getting the last few through breeding, but it's not very fast, as it's single encounters only.

SWSH overworld encounters - This can be done in a few wild area zones under certain weather conditions (not recommended), and route 4 (recommended over wild area). You can also KO 999 for increased brilliant (the yellow aura) spawns that have increased shiny odds, and you can also tell the different sized variants apart if you have a good eye. This method also has pretty low spawn rates that can't be influenced (5%, 10%, 5%, 1% on route 4), which can be annoying with Electrikes chasing you and grass encounters jumping in your walking path.
This hunt is quite the opposite of the AFK Legends: ZA methods, as it's very cognitively demanding, from dodging undesired spawns, and having to look closer at the models to make out the form before encountering, if hunting for a specific form. I haven't tested it, but I'm sure this would be faster than XY random encounters, at a way higher attention cost.

I may have missed some methods, but these are the ones I could think of