I've done this so many times the mere mention of it makes my knuckles start bleeding. Just kidding, get the rear end as high as you can, enough to USE the breaker bar not just enough to look at it. Spray with good penetrating fluid. Make sure that you take a pick and REALLY clean out the multi-point bolt hole, your strategically placed / wedged wrench on the nut should keep it from turning while you break it free. Obviously, pulling down on the breaker bar is easier. For the really hard ones I have used a small sledge and very long brass punch to bang on the offending bolt head. Try to keep line of impact straight. A pneumatic hammer with a brass head might work if you don't mess up the 12 point bolt head but I've never used one. Long 1/2 inch breaker bar to get them loose then whatever you want. If you don't keep the 12 point straight you will strip it and then there is only plan B.
There are different quality 12 points. I use Hazet or Snap-on because other brands twist or strip easily.
Good luck