I don't think I have a kneecapper shotgun... might have stashed it in my "special gun cabinet" though
In the end I got pissed off and took a fatman to it's face. That showed it, though the speed of its death messed up the spawning of the legendary fellas, they decided to pop up randomly later
it's a random legendary enemy loot item so you might never get one in a playthrough. all i mostly seem to get in loot drops are legendary right metal leg pieces.
Kneecapper shotgun, couple shots that filthy monster was a sitting duck. Pretty much 1 shot kill if I got in its spray though.
Lobbed grenades after that.