It would be hard relying mostly on Marksman, at least at first, and you'd have to be really careful about ammunition, but I think it's quite possible to do it. You'd have to use a lot of hit-and-run tactics, possibly get Paralyze-happy when things go badly, and generally be prepared to work for it. Sneak takes ages to get up to a really useful level, but once you're there you should be alright.