This achievement is still technically possible, can personally confirm. I was able to unlock it today, January 8th 2022, during mod 21.
This guide is relevant if the Skirmish hasn't been patched/rebalanced. This achievement has been almost impossible to unlock for years as of this writing. I don't think there's been anyone listed under recent unlocks in the last 6 months, other than my team. There's very good reasons for that - Cryptic's changes and inattention is forcing us to go to absurd lengths currently. But the difficulty could change. If there's a fix, sometime during mod 22 or later, then this guide is massive overkill. Please keep that in-mind - this is only for if nobody on TA has unlocked the achievement in months and if the gold rank seems unreasonably far off when you try for it, even with a good group.
This achievement requires 5 DPS with endgame builds, literally. You can't have a healer or a tank with you, the DPS loss would make the achievement impossible. Roughly 55k-60k ilvl (for mod 21's meta) average with mostly best in slot gear on everyone, all built focused purely on maximum damage. If you bring one person with, like, 50k ilvl made up of not-best in slot gear, the achievement becomes more-or-less impossible. The margin of error allowed on waves 1 and 3 is extremely small - if you get 1 or 2 less portal spawns in wave 1 from people getting kills a half second too slow on average, or if the wave 3 boss dies in 9-10 seconds instead of 7-8 seconds, you'll get silver instead of gold.
Wave 1, every spawn needs to die fully within about 3 seconds of it appearing, and the team needs to strategize on which encounter powers are used and in what order to cause that. Just perfect gear isn't good enough. What seemed to work best, was using your longest encounter cooldown timer by clocking right when spawns happen in relation to it, making sure to use the ability at the same point every time so it's reliable, to keep track of right when spawns will happen. Then, dropping any damage over time AOEs a second or two before the spawn, any 1-hit AOE the moment of the spawn, then immediately a single target encounter on the balgura/whichever is the beefiest enemy for the portal, so everything dies together. Each portal's spawn timer is individual/not connected to the others, and doesn't reset until every enemy that came out that portal is dead, so the goal is to kill all of the enemies from your portal as fast as possible. If you need to use 1 or 2 at-wills to finish off the strongest enemy, that will be too slow and you will get a final silver rank.
The trick is 1 single target encounter power for the biggest enemy, with 2 AOEs to kill everything fast, dropping all 3 encounters as soon as they become effective. 1 person per portal, don't run around at all, other than for grabbing the debuff purge when the blue ring is close to you. The blue screen, can't use powers debuff happens at 15 debuff stacks - the little red buckled over in pain guy icon with the number next to it. If you quickly go in and out of a blue ring each time it rotates close to you, you'll only ever get to 8-9 stacks and no actual debuff. If anyone somehow gets the actual debuff right as their enemies spawn, the 5-15 seconds to go grab the ring and come back and then kill will essentially void the achievement and you should restart.
Person 5/the lowest DPS supports whoever is taking a split-second longer than everyone else to completely clear their portal. This wave needs to end as essentially the highest silver possible, or no gold, a little over a 2/3rds full bar while at silver.
Wave 2 will end no matter what exactly half way to gold (or when time runs out) - it's broken. But it's easy to max out at the broken half way to gold amount. Just do the same everyone has their own portal strategy, run around and collect the essence from other portals every 2-3 spawns you kill, cross through a blue ring if it's close while you do it, wave 2 has a lot of leeway to mess up and go slow - not at all required to optimize like you do for waves 1 and 3.
What's important is how you use this wave to prepare for the perfect execution required wave 3.
Collect up essence and kill enemies until there is a minute left, then everyone goes to a different pillar (with one double-stacked). Before dumping in your essence, everyone needs to change powers mid-Skirmish, each AOE encounter changed to a single-target encounter, before you go into wave 3. The last boss will not die fast enough if all 5 people do not all have 3 single-target encounters, which need to be changed to before the end of wave 2 by pausing, going down to powers, and equipping new ones manually.
Everyone changes from AOEs to single targets, everyone then loads their essence into their pillar, you should end with a silver rank and the bar half way full. Everyone should automatically get their debuff stacks purged by the blue ring landing on them with the amount of time you'll be standing there loading in essences, so you won't need to think about that extra variable.
Once the wave end message appears, everyone needs to immediately reposition to where the final boss spawns, and be standing almost on top of the spawn before the wave 3 cutscene activates/before the boss spawns. If you aren't all 5 already standing close to where the boss spawns as it spawns after the cutscene, the second or two of movement where you could be damaging but aren't will fail the gold, it causes wave 3 to be too slow. Being really careful about all being as close to the boss spawn as possible as wave 2 ended, reacting and moving quickly, was the last strategic change needed for my group getting the gold. This wave needs to be not just gold, but a really, really high about as perfect as technically possible gold where only a few seconds pass from the cutscene to the boss dying. Drop all mount and artifact powers, all dailies, then all encounters. The boss should die in about 7-8 seconds after the cutscene.
Pull that all off and you can, juuussttttt barely, still get gold. Get a feel for it, don't get frustrated, strategize, think about every little thing you're doing then eek out every second of efficiency where you're able, and along with the required builds, you'll get it.
Of note, for gold, the rank bar at the end doesn't need to be full, just almost full. A really close to gold run's bar will be indistinguishable from a gold run's bar. You need the bar overall about 95%-ish full, for it to be a gold final rank.