How did the Female Titan cut Eren's titan form? 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?
See page 33, Chapter 32, "Compassion" from the manga.

The previous page followed discussion over Annie as a suspect in being the Female Titan. Mikasa asks Eren, having fought the Female Titan, if anything reminded him of Annie at all. Eren then remembers Annie's stance as a parallel to the Female Titan moving from that same stance into her kick. 

The stance resembles Annie's style demonstrated in Chapter 17: Illusions of Force

The text is spoken by Mikasa to convince Eren that deep down he knows first hand how close to the similarities are.