This is all my personal opinion based on my research of the fish. Take it however you would like.
It will need at minimum a 180gal tank, the longer the tank the better. They need a lot of room to swim and a lot of rock work. They also require a TON of flow. They live in an area of lots of rock with lots of crashing waves.
Purchase it from LiveAquaria. LiveAquaria spends an exorbitant amount of time QTing all of their specimens so that the casualty rate is very minimal. Many places will not even guarantee Live Arrival for an Achilles, let alone a 14 day health guarantee. In my experience and research of the species they do not transport well. They also don't live long if not provided with the proper conditions.
The best acclimation for them is a drip acclimation in at least a 5 gal bucket. Put aside a major part of your day just for acclimation and introduction. They are extremely sensitive to water change conditions, not to mention stress from shipping. 3 hours minimum for acclimation with a removal of roughly 1/3 of the water every 45 - 60mins. Minimal light is another key factor in acclimation.
DO NOT put them right into the display tank. While they need a large tank, with TONS of flow, to live happily in, a QT is of the utmost importance. Every fish/coral/invert you get should get a QT period, that isn't always possible. An Achilles however is MANDATORY. There is no question about it. No, well what if I do... QT the fish to give it time to adjust to the new water, shipping stress, etc. When you go to move the fish from the acclimation container to the QT, do NOT use a net. Gently dump the entire contents of the acclimation container into the QT tank with the fish. Nets add more stress than needed to the fish at this time
Prepare the tank with water taken directly from the main display. Also add live rock with at least 1 cave that is large enough for the fish to hide/live in. Leave enough space for the fish to swim the length of the tank. Make sure there is plenty of flow in the tank and your best bet is to use a good hang on filter like an Emperor 280/400. Use carbon cartridges and standard filter media cartridges. Yes it seems like over-kill but the water needs to stay pristine while the fish is in QT.
Do a 10% water change with water from the main tank every 24-36 hrs.
The fish should live in QT for a minimum of 3 weeks. This will give it time to adjust to the water it will be living in. It also gives it time to start eating different foods. They need to be provided with a varied diet. Nori(or similar seaweed/alqae) Marine Cuisine, Emerald Entree, not sure if you have it readily available but Rods Food, etc. Every feeding should have both Selcon(3-4 drops) and Garlic(1-2 drops) added to the water the food is soaking in. These additives help boost the immune system of the fish and provide them with nutrients that may be missing due to not having their natural diet.
Here is mine which is currently in QT