So, long story short, I'm not sure if the "Slowed not stopped" achievement in the new Lep Seclusa dungeon is buggy, instructions are unclear, or we're not the smartest tools in the shed, because we were looking kind of dumb with our finger and our thumb in the shape of a missing achievement in our forehead.
In any way, @ZOS_Kevin, in case you can add some insight ^^
First, so no one has to look it up, achievement screenshot. It's an individual achievement, not a group one

Now dear forum reader, Slow Descent is a debuff that you get when you get hit by Orpheon's cone shaped cleave when he light attacks. If I'm correct, you only get this debuff while not blocking. Debuff lasts around 3 seconds per hit, and I guess it drains your resources a bit. I can only guess because of the achievement phrasing, because the resource drain in veteran or hardmode is barely noticeable, maybe a bit more.
Maybe I'm wrong on the way it works, but you'll see how it's not that relevant. However, a couple friends and I are already Slowly Descending into madness.

So far it's pretty straightforward, go Lep Seclusa, and kill Orpheon after having your resources reduced below or equal to 25% in vet While afflicted/Because of by Slow Descent
When one checks the Help section in game, resources are described as magicka, stamina and health. But you can also consider ultimate points as a resource.

Now, we did half a dozen? attempts in two days, and two people got it, out of the five that tried. First, we all tried burning our stam and mag, getting our hp quite low in the process.
The person that got it was as clueless as we were, she swore she had done the same, drain her stam/mag, not paying attention to HP nor ultimate points.

The day after, we went again, 3 people only this time, one new member. The one that hadn't tried the day before got it on the 1st try we did that day.
After a couple more tries, the other 2 group members still didn't get it. And we checked Combat Metrics graph feature, because our brain got smart but our head got dumb. These are two parts of my stats on last try, a regular veteran Orpheon kill waiting at the start of the fight while doing this kind of minigame.

Slow descent uptime debuff on me is shown on the top of the graph as a purple bar, after being selected on the debuff In section, and all resources can be seen with exact numbers. At this point I had no more than 20% on any of them.
Because the person that got it with us told us she possibly dropped her ult on the previous try, therefore minding ult points as a resource, but not getting her HP below 25%, or not on purpose, I did go again for that in this same try. Again, same format, you can see how I'm under 25% on stam, mag and ult for the 6 seconds of the debuff.

Now, if anyone that has gotten the achievement, or even tested in PTS can help, we'd appreciate it. Regardless, some clarification from official sources would just be lovely, both if something is not working as intended and on the wording in general, even when it's a "just get good" in a polite tone.
I would also accept an addon that tracks the conditions and tell us "you're good, kill that fella" after we're guaranteed to get the achievement, since we got all achievements but this one (and Champion) in Lep Seclusa. It just feels wrong two trifectas in Exiled Redoubt took us less tries than a plain vet achievement. But I guess all that glitters is gold, only shooting stars break the mold.