yeah these things can suck hard to undo if they have been in for a while.
you need to get a long breaker bar onto it, but it is essential that you have it exactly 90o to the plug otherwise it will round off. If you need to use an extension to do that keep it as short as possible.
you may then need a long extension to the breaker bar to get enough leverage....but if you are doing this on the ground/stands there is often not space.
Sometimes on tough bolts you can heat them with a gas torch which helps loosen them, but not a good idea with a filler plug.
but frankly....it might just be easiest to take it to a shop and get them to do it on the hoist, you can spend more breaking and fixing stuff than it would cost for someone with the right tools to undo it....