After doing some...never again. My time is too valuable to try and scrape value out of rusty northeast subarus. In Oregon it's probably surface, light, easy. In the northeast, usually a rust spot means poke your finger through it and surrounding paint, rubber trim, internal metal all disintegrate to a hole 4 times bigger than you thought.
We don't know how bad it is - if it's just a small hole or the corner section - cut out another section, use a punch flanger, and weld it in.
Nibbler is nice for chewing through sheet metal or an angle grinder, dremel, sawzall, which you'll probably need to get the hidden sections cut anyway if needed. Cut it larger than the area you're replacing.
A punch flanger creates a step so that the edge of the new piece slides behind the existing metal, giving you two layers of metal overlap which saves time trying to get the metal exactly the right size/shape/orientation, you don't have any gaps to fill with body filler, and provides a little more heat sink for welding. But you can get away without one particularly if it's a small one off job.
A welder, not too hot or you'll burn through it. Spot weld, you don't need a continuous seam.
But really if it's bad it's a miserable job - it's often much worse than you think - once the outside sheet metal is off the rust behind it can be atrocious - falling apart, rusted all back into the crevices and folds and joints that come together, it's untreatable and you can't get to it all.