It's correct if you change 'peace' to 'piece' [these have the same pronunciation] but, in my view, it's not natural. The expression 'it's a piece of cake' is typically used in response to an enquiry about how easy or difficult something is, but not as an 'upfront' statement before the question has been raised. It's not wrong to say this as you have written, but to my ears it sounds slightly odd.

She wants me to do this stuff because she thinks it would be easy for me. She’s wrong. (‘Actually’ would most likely make sense if you were talking to HER, not me. ) For example: I know you think it would be easy for me, but actually you’re wrong.

Your sentence is mostly correct but has a couple of small errors. Here's a natural and polished version: Corrected Version: "She wants me to do this stuff because she thinks it’s a piece of cake for me, but actually, that’s not true." or "She wants me to do this stuff because she thinks it’s a piece of cake for me, but she’s wrong." Key Points: "Piece of cake": The idiom means "very easy," but note the correct spelling: piece, not "peace." Ending options: Both "that’s not true" and "she's wrong" work naturally. You can choose based on your tone preference: "That's not true" sounds more neutral. "She’s wrong" is slightly more direct and assertive. Summary: The corrected versions sound natural and convey your point effectively! 😊

* piece