The witch bride can be found in "The Passing" chapter 1. You can play through the campaign in Easy difficulty, and then when you see the bride, vote to change the difficulty to Expert and crown her with a shotgun.
Note that in Expert, you MUST headshot her, and ALL shotgun pellets must hit. It is doable with just the tier 1 shotgun, which most of the time can be found in the apartments leading up to the witch. If you are lucky, you might find a tier 2 shotgun, but it isn't necessary.
Also note that she must die in ONE shot, regular shotgun spam kill doesn't count. You can practice crowning witches in single player by enabling god mode and spawning witches.