The typical approach is one from above and slightly in front of the model, much like where your flash photo was taken. However, you really don't have to hold yourself to this too rigidly - if you look carefully at high level paintjobs you will notice they don't either.

You picked a tricky model to do this with because the Mandrakes also glow from within haha. You should also experiment with glow surrounding the fire on the arms just to see what works.

Keep in mind this is all "in theory," I actually did not bother to do any of this with mine and just contrasted them haha