Yamaha OEM drain plugs are the worst. You are probably missing the crush washer between the bolt flange and the case and it has galled. If you can get it to move just a hair, it will break loose. Try drilling the head only off the bolt with a 1/2" bit, then see if the shank will thread out from the hole.
The last one like this I removed was from a 98 YZ400F. We had the engine out of the bike and on a die cart, one guy with a torch heating the case, one guy with an extractor and hammer, and one guy with a punch and hammer. It took us a while, and it finally popped loose and was only finger tight the rest of the way out.