If you have already broken teeth without out a handle extension, and the wrench was kept flat (as you stated), you will probably just break more teeth, easier this time.
Your best bet is the PRI or other well made wrench that engages at least half of the teeth. But considering that you have broken some already, you will probably want to replace the nut.
Heat could do the trick, but don't get too generous with the torch. If that doesn't work, splitting the nut is the next step.
Carefully cutting thru the nut with a dremel or other grinding tool, and then retry the wrench. If you have cut enough of the nut, it will crack open and spread enough to come loose.
Remember, you do want to replace the nut now anyway. Also, accidentally making light cuts on the receiver threads won't ruin it