Except Alolan Muk is Poison/Dark, so it also receives Super Effective damage from Fighting.
[Edit] That awkward moment when you don't realize that Poison resists Fighting. So then yes, you would be correct, a Wonder Guard Alolan Muk holding an Air Balloon would be immune to almost everything. [/Edit]
The closest to an "invulnerable Pokemon" is the almost impossible to attain Sturdy Shedinja. Due to the interaction of the ability Sturdy and Shedinja's single hitpoint, direct damage attacks can never cause it to faint. If it is holding the Safety Goggles, it becomes immune to Weather damage (Hail/Sand). If an ally uses Thunder Wave to Paralyze it, it cannot be afflicted with Poison, Toxic, or Burn, so it cannot be knocked out by damage from status effects.
Assuming you jump through all the hoops to create it, there are only 3 ways (that I am aware of) to defeat the Sturdy Shedinja. First, steal it's Safety Goggles with moves such as Thief, Trick, Switcheroo, etc during Hail or Sandstorm. The second is to replace Sturdy with another Ability, through varios means like Simple Beam, Entrainment, Mummy, etc. The third is with the moves Perish Song and Destiny Bond.
The only questions I have for it are whether Future Sight delayed damage, and Struggle recoil will respect Sturdy.