He absolutely can be here for dealing with the Witch! I'm still thinking on some of the finer details of it, but methinks she'll behave a lot like a ghost - ie physical attacks won't be terribly effective (or have much of a chance to hit at all), and water is definitely out, but fire can be good to clear the globs of black water and do some damage, and the biggest weaknesses would be lightning and direct spiritual attacks - ie exorcism/spirit banishing spells.