Streets of Rogue - How to Kill a Ghost
There's only one way to unlock Streets of Rogue's Werewolf character, and that's by killing a ghost. Unfortunately for you ghosts are ethereal, and as such normal weapons won't work on taking them out.
Ghosts spawn in graveyards (as they should) and generally don't want to have much to do with you, but if you destroy their gravestones they get angry pretty quickly. Once you destroy enough gravestones the ghosts in an area will become hostile and you can take them out, provided you are prepared.
There are currently two ways to do damage to ghosts.
The first method is via using Electro Pills on ghosts, which make quick work of them. This can be done with any character but there's no guarantee you're going to get Electro Pills in a run.
The second method requires shooting them with a Ghost Gibber gun -- which the Scientist character starts out with, luckily enough.
How to unlock the Scientist
It's time to get the Scientist unlocked if you haven't done it yet, for easy ghost-killing.
You unlock the Scientist by polluting 5 air filtration systems. If this sounds tough to do on a single run, don't worry. You can pollute 5 of them across multiple runs, making the Scientist surprisingly easy to unlock.
If you have a consumable in your inventory simply walk up to an air filtration system and interact with it. Place the consumable in the filtration system and voila! You've polluted it.
Do bear in mind that the effects of the consumable will be emitted from the air from the filtration system. Keep that in mind before getting yourself killed.
Once you've polluted 5 systems you'll have the Scientist available for your next run. Get that Ghost Gibber ready and find yourself a graveyard to get the Werewolf unlocked. It's really easier than it seems written out here.