It involves Slayer Change and Mirror Image.
In fact, if you use Mirror Image and Slayer Change, you won't suffer any damage from Slayer form, meaning you can virtually stay in Slayer form all day.
It appears to work with every source of mirror image, namely DMM special ability, spell, scroll, or item (ring of duplication)
It does not make Slayer form OP given how bad it is on its own but has nice roleplay possibility, like, you mastered your essence and thus your Slayer Form, and can keep it as will.
And potentially good powergaming possibilities: think about what a swashbuckler could do in such a form.
Well, I may well do a playthrough based on this, it sounds fun.