。
She's harsh only with me.
Literal
She [topic-は] me [only-だけに] harshly hit.
つらく当たる ('to treat harshly,' literally 'to strike harshly') is an idiomatic phrasing for treating someone badly — using 当たる ('to hit / strike') metaphorically for the way someone's words and actions land on a target. つらい ('painful, tough, harsh') in adverbial form intensifies the harsh quality. だけに ('only to') focuses the asymmetry: she singles me out for this treatment. The は marks a contrast with how she treats others — implied but unspoken.