
On the other hand, xBRZ can produce smoother sprite edges yet does little to enhance image detail:

So I thought that combining both methods might produce an overall better result. The obvious solution is to keep the external outline of the xBRZ sprite and impose it over the version produced by waifu2x:

Clearly not perfect but rather fine I suppose. I kept an outline 3 pixels thin which I produced by first discolouring the xBRZ sprite to just a shape of single solid colour and then gradually removing the three outer pixel outlines (I suppose there may be a faster way to achieve this but I'm not aware of any at the moment).
I guess that the outer shape may be further refined manually but I haven't tried that.
For waifu2x processing I used this implementation, and the xBRZ Scaler Test tool is available here.