It's a split.
If Apocalypse can get even a split-second drop on her, she's dead.
HOWEVER...
If she can get a defensive spell up in time (Tcetorp em morf sih srewop), then start speaking backwards in tongues more than Linda Blair in the Exorcist, Apoc could be in serious trouble.
Context is everything. Saying Sersi can defeat Zatanna, while still being fearful of Apoc (as alluded to), doesn't mean Zatanna cannot defeat Apoc. Her human frailty, however, is definitely a key point in all of this...
http://img8.imageshack.us/img8/108/81108889.jpg
He doesn't seem to notice BB's scream.