Unsouled

For this achievement, you have to crank the difficulty up to Master Soul (the highest) and take out Kamas with a counter, without being countered in any way, and countering him a total of eight times during the entire fight. The eighth counter can also be the killing blow. I've included my successful fight below.

Kamas has a special bar over his head that swings between green and red. If you deal damage while it's in the red (even with a counter), he will auto-counter you and negate the achievement. He will also sometimes guard, and if you strike him it'll register as a JustGuard and also negate the achievement (seen at 2:26).

You also have to be aware of the size of the green meter. As you can see at 0:16, he'll start to charge and shrink the green section. Other things that'll shrink the green/grow the red: countering him, using moves he can't counter (Thousand Demons, Death Touch, etc.), or him activating his Active Soul. But if you strike him, it'll undo the increase in red a little with each hit.

I suggest getting your counters first thing, while the green zone is at its largest. Pick an attack you're good at countering and try to counter when the marker is just coming out of the red. If you Chain your counters at this moment, they'll all count, you could get all eight on one Chain, and the marker should still be in the green as you disengage. If you don't feel like risking it, then you could just get one or two at a time. You want to minimize the damage you're both taking while getting your counters; if he gets too weak too quickly, he may start his charging near the end of the fight. If you're only at a few counters and he does this, you risk killing him when you go to damage him to undo those effects.

For his attacks, he has a few you can counter: a repeated sword dash, regular sword swipes when you're close, and a long-range swipe he does. I personally liked the sword dash for my counters. The attacks you need to watch out for are a grab he does (he'll raise his hand and it'll glow yellow) and a dash where he turns into a cloud of sand. Both he likes to spam, can't be countered, and he'll often rush to you before initiating these attacks. You can actually hit him out of both, but it's risky and I'd suggest just dodging away. He does have two larger attacks (a cinematic grab and an aimed spin dash) but he doesn't use them often, and the latter is very telegraphed.

For Active Souls, I liked to use Soul of Bagoe to start to make it more manageable to get your counters. This is the soul you get from Kamas himself. From there I switch to Soul of Iskuz (Niug) so I can limit the damage I take as I whittle his health bar down. Lastly, I switch to Soul of Lusei (found in Act 4, netting you the I See Dead People achievement) to top off my health if I get too low.

The whole fight is all about taking it slow, minimizing the damage you're taking, and of course racking up the number of counters you need. Be patient.