He is called lone shadow yet there is another purple assassin with him. Fromsoft need to change his title for Ganker Shadow.

This boss is optional, it shouldn't say it's mandatory.

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Easy-ish strat: Sneak into the room, hug the left wall. Sneak deathblow the one-arm ninja dude, and use Puppeteer Ninjutsu. Try and stay behind Lone Shadow, whittling down his posture before you can deathblow. At this point, the purple dude will die, so just prepare to spam parry. Got him on 2nd try

Tip: Execute the guy on left side then quit the game to main menu, load back to the game, after that you will be able to do one sneak deathblow on mini-boss.

When ninjutsu puppeteer is about to end, you can perform another backstab and reset the timer by using another ninjutsu on him.

Easy strategy is to execute and puppet the guy on the left. You want to stay far from the right keeping your ally far on the left and the boss in the middle like a triangle. If you do this properly you should get aggro and the puppet should hit and the aggro will switch from you and him constantly. You only will get one or two hits in each time and let your friend hit and get aggro. At some point if you are playing aggressive enough you might get a knockdown or finish him to execute the first phase. if you puppet your friend again he should survive the a lot of the 2nd phase with the same strategy. Mind you my run went well where the puppet never got hit and the boss was constantly being stunned or deflected so it went quick. I used one gourd seed (estus) and I didn't even need to. TLDR: Puppet the guy on the left, stay to the right of the boss switching aggro with the puppet. Backstab and puppet your ally again at end of phase one and use same strategy for same 2.

if he didnt gank me, I would give mad respect to him. This motherfecker only has one arm and still kicks ***

once you beat this guy, you can just rest and then go and backstab the purple guy and rest again over and over. he gives like 300 something xp every time. easy way to grind xp if you were so inclined.