From Protector of the Grey House:
Natalia told him that she often had to infiltrate groups of Slayers. Before, it was one at a time. Now she often spoke with three or four at a time. She told Vincent she took care of the Slayers cautiously, making sure to kill them one at a time, when they were by themselves, instead of going against them all at once. Still, she had started training for the inevitable. She was fighting his vampires two or three at a time. Vincent didn’t care for these sessions, but allowed it, knowing she needed the training. It didn’t help that Joseph and Mierka both thought she was doing a worthy job and helped to push the human to her limit and beyond. Joseph fought her often and Mierka gave her pointers on stealth tactics.