Obsidian dragon yugioh?

Ok I have a defense position obsidian dragon whose effect is that it negates and destroys any spell or trap card that specifically targets one face up dark monster and I also have a summon skull, I attack with my summon skull and my opponent tries to activate dark mirror force that destroys all defense position monsters on my side of the field when it's opponents monster attacks....here is the question----is dark mirror force destroyed because it is targeting my dark summon skull or does it work because it is destroying the defense position monsters on the field and not summon skull