A verbal humiliation stronger than just the usual 'little slut' or 'whore' is to tell her she is beneath sluts and whores. I sometimes laugh at her and tell her she couldn't possibly be as good as the worst whore in the city.

"What, you flunked out of slut school?"

"You better put your heart into it, you lame bitch, or I'll find some other retarded slut who's better at sucking cock than you."

You get the idea.