You could say 넌 참 [정말/너무] 나빠 (You're so bad) with a playful tone. Some other expressions: - 너(는) 정말 못됐다: You're so bad [no good]. 못되다: bad. ill bred. - 너 참 잘 한다: You are doing great (sarcasm) - 너 정말 대단하다: You're so great. (sarcasm)