Bulwark is a boss in Final Fantasy X-2, the third one in the final battles against Vegnagun. It counters all attacks either with physical or magical attacks. Both Bulwarks will be revived when KO'd by the main body, so it is advised to ignore them in favor of attacking the main body.
However, do note that when a Bulwark is KO'd, the main body must waste a turn to revive it, therefore giving the player a slight advantage over it. It is recommended to have at least one character attacking the main body while the other two attack the Bulwarks if the player decides to take the strategic route.