Stripped hex bolt
I found the solution to breaking a slightly stripped hex bolt free from my stem from other posts on the subject (Soak the bolt area with WD 40 then tap a new, high quality, hex wrench with new flat sides into the bolt with a hammer and turn with downward pressure.) but I wanted to share another tip.
Turning the wrench was hard, so to increase the leverage I drilled a hole about the diameter of the wrench into the end of a piece of scrap wood and slid it over the wrench end. Now I had a handle about 2 ft. long and the bolt broke free with little effort.
You made a cheat bar.
There are any number of ways to do what you describe. A vise grip or an appropriate sized pipe are frequently used to get that extra leverage.