ERIC TB said:
If the foam is hard it's easy to do with a demolition/sorting grapple on a excavator.

That works also!

Do you know if the foam is held in place by studs or is it simply adhered to the surface? I you've got studs (they will probably be no bigger than a small wire) your best bet is to go at it with a large hand scraper. If no studs use a hoe or roofing scraper.
Also how thick is the sheet metal? Is it laying flat or is it vertical? If it is laying flat and reasonably robust, you might be able to use a roofing cutter. We use these to remove built up roofing for abatement or prior to demo to reduce fly debris. It's kind of like a mower with a verticl blade. You would still have to scrape it. A skid steer loader with a straight bucket would work for large flat areas. Vertically you could use a loader bucket (no teeth).
Also remember that the foam will need to be disposed of properly, especially if it is hazardous. It's always a good idea to get it tested for asbestos.