A Guardian's physical defense was ranked second among all the jobs in the game. It was only behind ranked number one: Templar.
However, a Guardian had specialised shield skills and was proficient in their skills. From a tank's perspective, a Guardian was entirely worthy of being number one.
Moreover, a Guardian had an absolute advantage when coming up against an Archer.
After level 20, Sharpshooter would learn [Pierce] which made them capable of penetrating any armour regardless of how thick it was. However, shields were considered weapons and not armour so it was able to block off Archers' arrows completely.
An impressive shield like Crotch Lord's Shield of God would be capable of defending two waves of arrows from top Archers without any pressure.
While the shield in Dark Moon Soul's hand was evidently not as good as Crotch Lord's one, it was still a peak grade shield. It was definitely more than enough to deal with Yang Nuo's arrows.
