I would suggest a Winchester Model 70 calibre .458 Magnum.
It is, technically, an "elephant gun" and with a good aim and some bad ass ammunition, will simply smash huge holes in the wing - thereby knocking all of the rust off........
More seriously, I have the same problem on my ML.
Plan in summer is to:
1, grind/sand off rust and coat with a rust preventing primer.
2, use light grade wet and dry paper to smooth off
3, get a can of matching paint and layer up
Needs a garage and a free warm weekend.