Spartacus is wide open to all attacks, though; not only he doesn't wear armour, his entire fighting style as a Berserker revolves around getting damaged and hitting back. His EX-Rank Endurance doesn't seem to mean he has invulnerable skin (well, not any more than action anime characters in general), it means he has a boatload of Hit Points because he's getting +7 HPs per Hit Die.