I use a mix of 50-50 ATF and acetone as a penetrating oil. Squirt it on and leave it overnight. Repeat as necessary.
Also, heat applied to the area the bolt screws into, or the nut if there is one. Repeated heating and cooling can sometimes loosen it up a bit. The penetrating oil mix will burn readily.
Once the bolt head is stripped you'll need another way to grab it. Before drilling it, see of you can get a nice big set of vice grips on it. It may help to grind a couple of flat spots on opposite side of the bolt head. If that won't work, the the drill is next.
Once you drill the head off then you no longer have the choice to just close it back up and forget it.
If you drill the head off you may be able to get the part out of the way and have easier access to use the penetrating oil and heat.