It looks like the brass stem unit has two flat surfaces for a wrench to grab, the rest is round.
It also looks like someone already tried to grab those two flats, but the brass is getting damaged and rounded over.
If a Crescent type wrench is used, it has to be as tight fitting as possible, or it will slip and cause damage like that.
An open end wrench is better in a situation like that, with a tight stem unit that needs to be wrenched on very hard.
At this point, you might need to use vice grip type pliers, or a big Channel lock type of pliers, to get enough grip to bust that stem unit loose.
Spray some penetrating oil where it rests against the dull brass body of the faucet, and wait one full day, then clean off the oil with detergent, then try again.