[catch] a Tartar

to get into a confrontation with a person or thing that is more powerful than what one assumed

The idiom "catch a Tartar" has its origins in the 18th-century British Navy. Tartar was used to describe the people from the Tartary region, who were thought to be fierce and difficult to capture as prisoners. Figuratively, the phrase means to unexpectedly find oneself in a challenging or difficult situation or to deal with someone who is unexpectedly formidable or troublesome.

It looks like we caught a Tartar when we tried to muscle their store out of the area.
He thought the assignment would be easy, but it turned out to be a real challenge, like trying to catch a Tartar.
The young chess champion didn't realize he would catch a Tartar when he challenged the seasoned grandmaster.
Dealing with the demanding client proved to be catching a Tartar; they had high expectations and a sharp eye for detail.
The new teacher thought the students would be easy to manage, but some of them turned out to be Tartars.