My character, a warpriest with the archetype sacred fist, wants to beat up enemies with his fists but ignore their bonuses to AC from armor and shield. Is there a feat, magic item, spell, or something else that will enable him to do this?
This can be achieved by a Brilliant Energy amulet of the mighty fists. Attacks with that equipped ignore all unliving matter, which lets you ignore armor and shields, but makes impossible to hurt undead and constructs.
As an alternative you could put the enchantment on bodywraps of mighty strikes, as noted by Frezak. As those need a +1 before you can put special abilities on them you'd need a +5 one. But as their base price is lower the difference isn't too big.
If your DM allows you to target your fists as weapons:
Dimensional Blade (cleric 6th)
This spell gives you the power to turn one melee weapon in your hand into a two-dimensional object with length and height but no width. A sword becomes a flat image of a sword, a mace is reduced to a flat outline, and so on. The magic of the spell allows the weapon to be wielded by you normally, creating just enough depth to grasp it.
Attacks made with the weapon are melee touch attacks that ignore all armor. Force effects, such as mage armor and shield, still apply their AC against attacks made with a dimensional blade. Bludgeoning weapons used to make attacks with this spell deal half damage and are treated as slashing weapons. If the weapon leaves your hand before the end of the spell, the effect ends.