Look, I get that with a pin-point accurate location targeting system, human heads can't be insta-kills, but it still breaks immersion that headshots do next to nothing.
So FOR HUMANS ONLY, I propose that if you disable the head down to 0, it automatically stuns the target for 1 turn. That way, targeting a human's head actually means something without completely breaking the game.
You would have to balance this with increased armour for helmets, but that's easy.
Shouldn't apply to the Pandas. They're obviously necro-zombies run by a viral hive-mind, so their head is simply vestigial.