Rabastan had felt the Cruciatus more than once. It was one of those hexes that he loved to hate. He loved casting it, loved the pain from it, but he hated that even he couldn't withstand it forever. He might enjoy the pain, but eventually his brain would suffer from it.... Very eventually.
He kept his own spells up as the first sensation hit him, nerves tingling until everything was painful, but it wasn't strong, not yet. Still, he ended his own spells, waiting for her to do the same.
"It's a very good first try. You're very powerful, but you don't embrace the pain. You have to let that pain fuel your anger, hate me, despise me, even for that one instant, and use all of that to hex me. Use your hate and rage to make you powerful. Use the pain to make you strong. Again," he said, just as he cast the spells again.