If you just want to get them the fastest possible there are many ways.

WP is still pretty good if the group is decent (like 15-25minutes für 50 Myth), the Brayflox first 2 bosses comes to mind too (not that I ever do something like that, but it IS efficient^^) and roulette also helps. The 100 Myth in CT is not super efficient, but you might get some extra loot for another class you're using etc.

Roulette certainly has it's downsides though. I love PS and am happy when I get it, but for Myth-purposes it's a fairly bad deal^^

Daily Quests and/or Treasure Hunting aren't super slow Myth either.