Just spent over on hour on the Spear General + the Sword guy. This fight is so retarded.
You have to be aware of the super aggressive Spear general but the space is small, there is fire and the camera is an abomination.
There is one rifle man which you can posses for some distraction but he doesn't live long.
My solution was to bait the sword guy to fall into the pit and then be aggressive with the Spear General. It worked but it took an hour for the stars to align.
Just cheese it, there's always a way to cheese the minibosses of the open maps. Aggro them, then lose aggro by getting to the branch and jump to the area that leads further away from them and the idol on the side. Then get back on the branch closest to them once they turned back, quickly jump off the branch and hang on the ledge, and use the finger whistle prosthetic on the sword guy. He will come looking for the source of noise (you), move a bit to the side and he won't notice you, then once he turns his back to you, you can quickly get a backstab on him, allowing you to possess him with the ninjutsu technique.
Now, instead of having two enemies, you have an ally fighting alongside you. Occasional firecrackers and careful hit and run while your ally keeps him busy and he's down in no time.