These are the methods i would try, in order of personal preference:
1. Fill in flash, if possible, ie if can reach.
2. Take two slides on tripod, expose one for highlight, one for shadows. (aka bracket) Scan and combine in PS. [Similar method: bracket like mad, take the few best ones.
3. Don't shoot. Wait for clouds, or sunset, or sunrise, until i know the slide can handle the range of brightness in the scene. That's about 5 stops at most, i think.
4. Use Supra400. Yeah, i know it's negative. But it's known that slides have higher contrast and lower dynamic range. On a bright/deep shadow shot, shooting slides and expecting highlights and shadows to come out in one shot is just not too realistic. At out present stage of emulsion technology anyway.