out of (one's) depth
In a field or situation that exceeds one's knowledge or ability. Of course she's out of her depth with 25 employees reporting to her—she had hardly any supervisory experience before you gave her the job! I asked Alex to consult on this project because it's a bit out of my depth.
*out of one's depth
Fig. involved in something that is beyond one's capabilities. (*Typically: be ~; get ~.) You know, you are really out of your depth in this project. I am sure I am out of my depth in organic chemistry.