After she killed the Levi squad, Eren transformed to fight the Female Titan. In Chapter 29, Hammer, it looked like Eren was having the upper hand, but then the Female Titan turned around, and with one horizontal motion, cut Eren's titan form's head in half.
How did that happen? That part wasn't clear in the anime nor the manga. Did she take a tree and smacked him? Did she harden her hand and hit him?