This confirms Emperor level > Mihawk level.
If Mihawk = 1 Emperor, then surpassing Mihawk is enough for being PK.
A guy who surpassed Mihawk, can take down Emperors 1 by 1 IF Mihawk = 1 Emperor but its not. Thats what Luffy is doing, fighting Emperors 1 by 1, not at the same time.
But since 1 Emperor > Mihawk, Mihawk knows being PK is more difficult than surpassing him.