。
I'm angry with her.
Literal
I [topic-は] her [at-に] stomach [object-を] standing-[state-ている].
腹を立てる ('get angry,' literally 'stand the stomach') is a set idiom for anger. The target of anger is marked with に. The ている form marks the ongoing emotional state. Interesting culturally: in Japanese, the belly (腹) is the seat of emotions, especially anger — compare English 'gut feeling.' The kanji 腹 figures in many emotion idioms.