Achilles is my dream fish and over the last 5 years I've sourced 2 healthy ones(one at a time) from LFS and they went straight to quarantine. Made a mistake in not treating on arrival as they seemed healthy on arrival and ate on day one. On day 8-10 they both got velvet and even though I started copper treatment on sight of white spots they both died within 2 days.
Ordered one from a reputable wholesaler which arrived supposedly quarantined, and well fed and it showed on how plumb it was. It also ate on day 1 and did well for few days till I saw body erosion for days. Did copper treat as well as antibiotic soaked food and it continued to loose it's skin and died a nasty death. The day it died I had decided to do a FW dip and I still did it post Mortum only to realize it had a nasty flukes infestation.
A year later bought one I had gotten to a friend and it had been healthy for 4 years only to loose it few months later to a weird skin eating disease that escaped quarantine and killed a bunch of healthy fish collection (lost 8 fish from my collection, needless to say the nicest, most expensive, most difficult to source)
A bad experience but it only shows how difficult this fish is. I have 4 quarantine tanks but the Achilles were all in 80 g Qtank. Well filtered with skimmer and loads of hiding places...
Plan for the new Achilles if any:
Restart Q tank with clean new water or use tank water but keep fishlessfor 72 days plus before getting the Achilles.
Treat with copper at full dose or 3/4 dose on arrival. Feed 4-6 times a day soaked in Vitamins.
Keep nori clip full at all time and change remains every 6 hours.
Water change 30-40 percent every 3-4 days correcting for copper.
Don't tell anyone it's here before its in tank for 2-3 months

It is a super delicate fish with thousands of pages threads on introduction plans and issues unfortunately most with sad endings that is why it's a dream fish for many.
Good luck with yours.