Is there enough ‘flat’ left to use a 6 point socket?
If it’s really that rounded, and you need some big leverage, I’d go to a pipe wrench next – it’s not like you are going to reuse it at this point.
That’s the first I’ve heard of being able to tighten an oil drain plug to the point that it strips the plug head and not strip the oil pan first. Maybe it leaked and someone used red locktite on it?