I have removed a lot over tight and stripped drain for myself and others. I use what I call the brutal method. hammer and blunt chisel carefully rapping it in the loosen direction works often for me. I have also used a brass drift and rapped on the plug end and that works some times. I have never had to drill one out. When I go back with the steel plugs in aluminum I put anti seize on the threads and tighten to no more that 12-14 ft lbs.
Then this is a time for a 1/4" drive impact. Something like a DeWalt or Milwaukee. Put the 8mm socket into the drain plug, Shove up as hard as you can and hit the trigger for a second or two.
Not loose? Repeat as necessary. If it still won't move...up to a larger impact. if allen socket is a good tight fit and you shove it into the drain plug and do your very best to keep things vertical...you should be able to hit it hard enough to come out...without tearing up the threads in the case.