I;ve seen some kighty odd things over the years when folks don;t want to fix something properly. It very possible that someone had a leaking plug and used a locktite type product on it instead of replacing the plug and seal. I would try a heat gun or other heat source first.
My method is to jack it up so you've got plenty of room for a long breaker bar, lay underneath and hang on to the crossmember (or rear axle depending on how tall you are) and heave on the bar with your feet. After all, the leg muscles spend most of their time holding your body weight up so you can get a hell of a lot of force from your legs.
a brute force method I have used (but not on the sump plug) is to put a spanner on it, and then put the jack under the spanner and start jacking. Make sure you are going the right direction. The only concern is if it is really seized up, you might break the weld between the female part and the sump. Plan B is to buy an oil extractor and pump the oil out through the dip stick hole. You can get them from marine shops, including a you beaut vacuum extractor type which saves burned hands.
I agree with using a long wrench or bar with a snug fitting socket. An adjustable wrench is no good; you'll just round off the head of the plug.
If you do jack it up to get more space for a breaker bar, be very careful about your position underneath - try to support the vehicle on axle stands or blocks so that any forces transmitted to the chassis when working on the plug don't cause the vehicle to topple off the jack. Working on a vehicle just propped on a jack is always asking for trouble.
Would I be correct in saying that particular sump plug uses an imperial box spanner/ socket? Nobody has mentioned freezing the plug. It works. But, if thread-lock has been used then a snug fitting tool an a lot of impact from something like a large gentle persuader (lump-hammer) might open it. IMPACT being the key word here.