What kind of metal is the thing thats rounded?
^^^ meh tried it mate and still no good
^^^ meh tried it mate and still no good
Have you tried putting it on so tight that you hardly can unclip it again?
If not, do so, wiggle it a bit, spray some WD40, wiggle again, WD40, wiggle, WD40, and so on.
Coca cola is good too, but it needs time to penetrate and work it's way trough the rust.
Or angle grind/dremel some bits of the nut so you can fit a wrench again.
If all of that fails, acetylene torch + vise grip
Just listing the possibility's that are going trough my head atm