Shadowdancer's Hide in Plain Sight works "As long as she is within 10 feet of an area of dim light". Complete darkness is not dim light.
Hellcat Stealth works "in normal or bright light". Complete darkness is not normal or bright light.
Without cover, this character can't hide in complete darkness from someone with Darkvision.
I think two different issues are being blurred here: hidden is unseen and unheard. Requirements to be unseen:
Being located in a heavily obscured area
Being located in a lightly obscured area only if you have the
Skulker feat or Mask of the Wild - you're not clearly seen in this case
Positioned where something is blocking line of site
It's hard to pinpoint one monster that does all of these things well, but here's some exemplary suggestions of each. (These suggestions assume that as a level 20 party, you're talking about true polymorph and thus aren't restricted to beasts).
Hard to beat the Invisible Stalker for this. It's just... always invisible. No concentration to ...
A creature that possesses the extraordinary ability termorsense "can automatically pinpoint the location of anything that is in contact with the ground," but the creature's tremorsense "range is specified in the creature’s descriptive text." Thus it's normally impossible while walking on the ground to use Stealth to sneak up on a creature ...
Under the Stealth skill it says:
"If people are observing you using any of their senses (but typically sight), you can't use Stealth." (PF 1st ed, Pg 106). So I suppose you'd need to decide if someone with Tremorsense counts as observing you, as Tremorsense states that the owner always makes perception checks if the target is in contact with the ...
Eldritch Blast's (EB) description states "A beam of crackling energy streaks toward a creature within range. [...]", so I would say it's at least audible in the near vicinity (the next room?). You could place a Silence spell on a creature before casting EB to absolutely silence the spell, as well as cries for help of your victim.
Unfortunately it ...
You point a finger at a target in range. Your magic grants you a brief insight into the target's defenses. On your next turn, you gain advantage on your first attack roll against the target, provided that this spell hasn't ended.
True Strike only has a somatic component, that of pointing at the target, so while that would be noticeable to all present, it ...
"Darkvision is the ability to see normally in total darkness." Seeing normally is what other creatures do in "normal or bright light". As such, Hellcat Stealth should work as stated.
Hide in Plain Sight, however, wouldn't work. In total darkness, you can't have brighter and dimmer areas, and as such, there is no way to have any shadow or ...
Don't hesitate to slag this if it doesn't count.
Looking around in Spheres of Might (which is 3PP), I found this talent:
Creatures using unusual forms of sensory perception such as
blindsight, greensight, or tremorsense cannot automatically foil your
use of Stealth; such creatures must make a Perception check as normal
to detect you when you make use ...