Yeah, you're right.
Bill's advantages are Stormbreaker (ie firing magic lightning bolts) and his massive battle experience.
Supes' only notable advantage is his speed - which in my opinion is above that of Thor and BRB. His normal advantages are of little consequence here as:
a) His (possibly) superior strength is negated by Bill's greater combat skill.
b) His normal invulnerability is negated by magic weapons and attacks.
I think Bill wins this 7/10.
And you know I must have some respect for the guy if I give odds like that AGAINST Supes lol!