I'm at the part of the Forlorn Watchman quest where I need to defeat the wraiths in the Emma May. However, they are proving far too difficult for my level 18 character. Can you all recommend strategies for defeating them (ie. spells that are particularly effective, or defensive strategies)? Or is it something I should wait a few more levels to try to do?

Fire or Shock work on them. They're immune to Frost and Poison, and to ordinary weapons. They usually hit you with Frost, so if you can absorb or resist that, you're better off (clothing or armor enchanted with Frost Shield is good here.)

There's not much point in waiting to take them on. Their health will continue to rise as you level up.