Once you defeat Mad Fiona she will disappear and reappear in a different form.
This form is called Golem Fiona. Kill Golem Fiona. (Hunting Rifle is good for this part of the boss fight)
Once you kill Golem Fiona normal Fiona will appear. Kill Fiona. During the time you are killing her a ton of witches will spawn. Fiona does not attack in this form she relies on the witches.
Once you kill her final form. Dialogue will appear. This is where you get a second chance to choose at either being a Vampire or Human. Depending pn what you choose it will still be the end of the campaign.
Once you finish the credits the server will teleport you to Harry to be taught your last spell.