Near the end of mission 3.4, it's possible to trigger a glitch that makes it completely impossible to initiate the bossfight. When you receive the objective "Defeat the Ambushers," it is possible to trick the game into thinking that an enemy is still alive by letting them die to bleed damage inflicted by your Nobushi companion. (Note: It may only trigger if you do this to the LAST ENEMY STANDING.) So long as you don't inflict the final blow, the game will not register the fact that you've killed all the enemies in the zone and the objective has been completed.

I'm not sure if this is possible to replicate in Knight missions where your Peacekeeper companion fights alongside you, but if such parameters exist for this glitch to be performed beyond 3.4, then those must be directly addressed in the next hotfix in order to avoid similar game-breaking and progression-stalling issues.

(Note: I've only been able to trigger this glitch once. I'll see if I can trigger it a 2nd time and if I can, I'll shadowplay a quick vid.)