First, a note: Extreme encounters are mathematically a 50/50 chance of a TPK (if fought to the death), since danger roughly doubles every two levels and enemy levels are roughly equivalent to player levels. Tactics, an extra player, and planning can tilt this balance, but I'd still reconsider unless this level of lethality is what you had in mind.
My favorite monster builder is https://monster.pf2.tools/ and the guidelines for creation are at https://2e.aonprd.com/Rules.aspx?ID=2874. I'd suggest picking picking a basic roadmap (in the builder), designing one or two core abilities, and tweaking from there. It's always a good idea to compare against monsters of a similar level to get an idea of ability power levels, and what trade-offs powerful abiilties typically have.
Some ideas:
- Knucklotec fists as a reach 15 weapon, and flavored as the fist flying to their target.
- The fists as minions to be commanded (more complicated, as you have to create and balance more than one creature).
- Depending on how mobile the main body is, potentially just an inert target, which the hands as the active creatures.
- Party actions are more numerous than enemy actions, so other mechanisms like hardness, resistances, or saving throws might protect the core more than a single reaction. Still flavored at your discretion.
- Depending on desired fantasy, the core could act as a spellcaster to buff the hands and debuff players.
A lot of that depends on now much independance you want the fists to have, because there's a whole gamut of options from intelligent trap, to solo creature with flavored abilities, to boss with minions, to three distinct entities.