This is the updated chart:
It is only useful for elemental characters (wizard classes, zealot), and only useful at PvE. Example: Wiz3 ele3 warlock3 build uses many elemental skills, so this card would be worth on this kind of build at PvE. A build without penalty (such as psychokino build) will not benefit.
Example: Dark damage vs dark enemy (penalty of 25%).
Mirtis card reduced this penalty by 10% of 25% per card level. (each star reduced penalty by 2.5%)
Mirtis card lv1 : penalty reduced to 22.5%
Mirtis card lv2: penalty reduced to 20.0%
Mirtis card lv3: penalty reduced to 17.5%
(…)
Mirtis card lv9: penalty reduced to 2.5%
Mirtis card lv10: no penalty with Dark vs Dark
From KTOS player guide:
Best PvP card: Blut. Useful PvP cards: nuaele, zaura, rexipher.
Worldbossing cards: marnox (physical), mirtis (elemental magic).
Best card to clear raids: pantorex (bosses are beast type) (not from worldboss).
Good PvE tanking card: dragon card (not from worldboss).
PvE best effect card at 1 star: helgasercle for farming classes (such as linker-thauma builds).
Least useful: kugheri balzermancer card (not from worldboss).
Kugheri and dragon cards are useful for necromancer necronomicon.