High difficulty, last stage in event dungeon is as far as i can go. So for anyone hunting the cards from chest or hunting treasure for exchange, which difficulty would you guys go to to farm those? Unless higher difficulty has higher chance to get the UR and SR card but then there is also more stamina cost.

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I guess most people will farm at High - 4 like you said. Higher difficulty dungeons have higher treasure payout per stamina point, but the Extreme level dungeon can be risky as you could fail to clear them and lose some potential treasure. There aren't UR Kirin in chests, only SR, and High level dungeons already drops them.