The Shadow Double isn't in the Bestiary; I figured it's just an extension of the Ninja Boss rather than a new enemy itself.
He'd be a lot tougher if you didn't have two people. I actually made him before I got around to making certain effects/monsters/characters guard allies that aren't at low health, so I had to make him raise his max HP way up to make his gimmick work.
On a related note, the next bugfix will prevent stair-related physics-breaking in Jelirol and Tor Torken. I've been putting blocking events in as they come up.