Get a farming build (with hirling) with 650% gold find and luck on both characters. In mythic 3 that means 1 crystal gold find on both characters, 1 greed nature on both characters and the fortune 20 on both characters for 650% gold find. Same thing with luck as what u do with gold for more space and efficient farming.
The most important basic thing in farming is item drops though. If you get fortune 20 on both characters, that's a free 100% item drops in total. The maximum item drops is 200% but with hirling a its 300% because of the hidden 100%.
That would be maxed 200% item drops +100% hidden item drops.
Either you get 3 legend item drops on both characters (expensive) or just use fortune mythstone on both characters until it reaches 300% item drops on adventure page.
For the mythstones, I just used the mythical hat anyways because it increases crystal rarity. Ascensions also help because accomplished makes achievements (feats) easier to complete, fortunate for gold, dealer for more gold and more. Mythical applies to every mythstone feat as well as the shop mythstones and finding mythstones.
How I found lots of high rarity mythstones was go to floor 500 (of any difficulty) and just farm there with mythical and item drops. Legend maps ofc help a lot too because they can increase mythstone rarity.
So far I got 1 elixir , 4 zeniths, 8 masters, 16 times,21 returns, 35 fortunes, 47 quests and then mythstones before quest I got 100s and for some I got 1500 wisdom and clarity. I feel like I never gonna run out of mythstone lol because I farm them easily now. Gold being easy makes everything easier too.
If your low on high rarity mythstones, do try to complete the convert mythstone feat as many times as possible and if possible, try to reach floor 500 on very easy mode and farm farm farm.