I've ran into several stuck drain pan bolts on both my own vehicles and friend/family vehicles I have worked on. Most of the time a drain plug costs $5 or so. Whenever I run across a stuck one that I can't get within a few minutes I usually will just go buy a new one if it's convenient to do so. Then I take a vice grips and a rubber mallet or large hammer and clamp the vice grips on tight and give it a nice whack with the hammer. I have never been unable to remove the drain bolt this way but usually when they have been in this tight they have already been stripped out and pretty beat up anyway so I would rather have the replacement plug handy before I start getting too rough with the stock one. If the drain plug head is completely rounded off you can sand the sides a little to make a flat surface so that the vice grips will grab better.
