The force of the spell had taken her by surprise as she found herself thrown back against the wall. As most spells were useless against them in this form, either too weak or too adapted to humans to catch them, this was was definitely more adapted to keep a werewolf at bay.
Laying winded, she saw Fenrir dart after Tonks, and then saw him fall back down the stairs. Snarling she got up, stopping by Fenrir long enough to make sure that he was ready to move too as they both headed up the stairs at the door the woman had slammed behind her.
Snarling again, Natasha lunged herself at the door, feeling it quiver but not break. It had to be reinforced by magic. No inner door in the house would match her strength in this form. She stood back, waiting for Fenrir to turn the door into splinters. Then there would be nothing standing between them and the woman. Lupin would pay for his betrayal, and for interrupting her fun at their last encounter.