Fighting this guy with 7 attack power, whirlwind slash does loads of damage to him. Just keep circling him to his right, whirlwind slash his butt, then chain axe combos on the brown one when he spawns. Surprisingly easy boss.

As many commenters have pointed out the two monkeys attack in a coordinated way, not on their own independent AI like other from duo bosses. It takes a little getting used to but you can treat a lot of their coordinated attacks the same as a long combo from a single boss. My favorite is when headless jumps headfirst at you with his non-perilous sweep. Jump over it and land one or two attacks. At this point brown is coming with her uppercut, parry it and headless's sword swipe immediately after. Now headless is coming with that overhead attack we know and love. Parry it, and right when you finish spearing out the centipede brown is midair with a leaping slam that you can quickly parry to finish the combo and feel like a bad, bad Shinobi. They do this combo attack often so take advantage. Treat all their coordinated attacks the same and you can finish off headless without trying to kill brown first, which I found more difficult on later ng+ cycles or charmless demon bell runs when she has much more hp.

Bro needs a dentist.

It's an optional boss and can easily be skipped.

Use ako’s sugar and divine confetti. For 1st phase, deflect until he does heavy overhead slam, use spear. For second phase, attack brown first, spam firecrackers, she will always get close when headless roars. Then it becomes 1st phase again.

In my first playthrough at these guys the brown ape was extremely passive until I tried to attack the headless ape which is when the brown ape would rush in and start flailing about. And I couldn't focus down the brown ape since the headless one was non-stop chasing me and doing long chain-attacks. Just kinda more annoying than fun if anything. I am running it with just my sword and the fan for screams though so who knows maybe it's easier with a more optimal set-up.

Alot more fun than the Sunken Valley ape, mostly because it doesn't run around for 10 hours while I try to hit it until it gets close and roars but I was too focused on hitting it and then I die because no umbrella which stops the terror effect and then I ragequit and then stop playing for 6 months then come back

The first strategy tip in this guide is just to skip the fight entirely lol

When you get to phase 2 of headless, i recommend spamming shinobi firecracker on the brown ape to bring the posture down and perform a deathblow

you decapitate it "surely its dead" wrong. you kill it when its decapitated "yes its dead". wrong. kill it again "yes just one more blow and its dead" wife comes to his rescue. now surely he is finally dead right?