If this is the hollow hex filler plug.. ?
Clean all around it with fine brass/wire brush and inside it.
Apply a drop of rust buster at the top so it runs down around the thread.
Leave for 20 mins or so.
Jack vehicle up a bit and block it/ axle stands & take a wheel off to give better access.
Apply another few drops, tap the hex plug tool well into it, connect up a 2' powerbar, arrange angle so you can put most of upper body weight on it, ensure tool is straight into plug, and lift yourself up on it and jerk.
A T-Bar might be better with weight on one end and someone ready to give other a hefty clout with a club-hammer as the weight comes on it. Even clouting socket end inwards as you pull can help.
I did once on an old box first use this
method then it finally loosened with a T-bar...
Made a bit of mess of the plug, but they're only a few quid or can be cleaned up with a
thread file
Or borrow a good battery impact wrench...
or a
cheap one
Just get a good new hex plug socket that takes a 3/8" or 1/2" sq. drive preferably and don't give up too easily. the impact punch method may not appear to do much, but with perseverance nearly alwasy will start it moving. In this case you might as well allow the punch to crumple the hollow plug inwards, which will help break the seal (i.e. progressively muller it!) Scribe it before you start so you know when you're getting somewhere with it!