Finished the first DLC, which was mostly decent and am now playing through the second and already the very first boss you fight, Gan Ning, who's just some random pirate, is so far my personal best boss in the entire game.
He fights like a regular human enemy but on steroids, he uses fists and has the largest variety of different red attacks of all the bosses so far and all of his red moves are the same martial arts you can use, but he can modify them in different ways (like launching them from the air or delaying them).
His moveset is purely reaction based rather than memory based, because he can follow up any attack with pretty much any other attack, including comboing 3 or more red moves in unexpected ways, which I really like.
He is fully staggerable (at least in the first phase), which means your deflects are much more impactful (and look more impactful) and you can safely use a lot of your martial arts.
Despite being staggerable, he can escape being stunlocked by using a grappling hook to pull himself to the other side of the arena (which he can also follow up by a dive kick red attack, or throw bombs around the arena, or throw daggers at you (once or several times), or throw another grappling hook at you, which you can deflect and if you do he falls out of the sky like a dumbass) or by jumping behind you, which he can immediately follow with a red attack.
He is one of two bosses who can deflect your attacks that I've encountered so far, the other being the final boss of the main campaign.
In the second phase he gets a stagger immunity aura, which makes the fight less proactive and more reactive, which I'm not a big fan of, but because he uses so many red attacks once you get good at his timings you can quickly deplete his spirit meter by deflecting them.
Not the hardest boss I've ever fought, but quite difficult and very demanding of your attention and by the time I beat him my hands were all sweaty.
I think this might be the best boss in all Team Ninja games that I've played and one of my favorite bosses in action games in general.
On a scale of 0 to 10, where 10 is the final fight against Vergil in DMC 3 and 0 is Bed of Chaos, I'd give him an easy 9.