Hello, the explosive witch can be found in the customize menu (on left side of screen), with the eyepatch icon (white crest on red eyepatch). I recommend applying the skin thru the bundle (hanger icon) to apply the body marks and extra effects (pull out explosion if full filled gauge, red eye lens flare on insertion).
This skin is available from game version 1.7 and above.
I hope this helps, and sorry for the late reply,
~NK