You then place a Puzzle Ring in the cube and open a portal to Greed's Realm, where you will sprint past everything until you get to the boss itself and just kill him (you can do this on Normal difficulty as well). Have a max level weapon ready for them with the Gem of Ease and the Illusory Boots power in Kanai's Cube. If you do have a lot of Puzzle Rings or are a solo player, you need a new character, but now you only need to level them to lvl 7. The snapshotting method mentioned in our guides could be a better option for you if you lack a lot of rings and have access to other players to farm, so you can check that one out here. Now while this method is fast in execution, it will still take a lot of prep time, as you will still need a LOT of Puzzle Rings and a Gem of Ease, but both of those you can relatively easily get through regular play and without wasting too much time on separate farming. The low-level snapshotting method has also been covered before, but there's a newer wrinkle on it which should result in players getting the items faster and being able to do it much easier solo.
While we've already covered the actual way to farm maximum level ethereals, there's a new and better method for getting low level ones to complete the Feat of Strength achievement that will allow you to use the items' transmog in later seasons/the main game after Season 24!