Hello friend, if you do the chapter XIV dungeon there is a good chance of winning this crystal. Anyway, it appears in several places, I already had one in adventure chests and in the new season too. Either way, farming Skullguard Keep at any level is the best option
Each hero class only receives missions from that follower that give a single crystal, if you complete the main story with a new hero you therefore have a crystal to win in the sidequests
While you probably know, I feel like I should mention one of the best ways to get more follower crystals is to complete the story and side quests with each class.
Yes it gets really boring, but it provides a lot of crystals.
It depends. If you want to rely on the gem requiring ability to get chosen epic runes/scrolls it is indeed better to keep him low so that he only crafts epic. But at high levels you can use his random rune or scroll ability to get a decent amount of epic runes or scrolls (at lvl 45 around 40% of epics, more or less 1k ore per random epic).
I used to keep him low but since I don’t want to spend gems on runes and scrolls I leveled him up and he is crafting a nice amount of them in exchange of my ore.
Craft random rune/scroll (green background). Requires gold and ore and gives you a rune or scroll of random color and rarity. For this option the higher his level the more epic runes/scrolls you will get:
Craft rune/scroll of type (blue background). Requires gold, ore and 100 gems and gives you a rune or scroll of the color you want and “random” rarity. For this option keeping Darkhunter at a low level, 25 for example, will make the rune or scroll you get to be always epic: