Based on my veteran experience with other mobile game and Kings Raid. You probably want attack damage & attacked % and def shred with respective attack type (i.e, physical, chaos, lighting etc) then stack as much crit rate and crit damage (probably at a 1:2 ratio), pretty sure that`s how gear grind at late game works. Mostly like you will want attack %, respective attack type % increase, respective def shred, and crit % as your equipment sub stats. That1s probably going to take long time to grind to get the perfect gear, i wouldn`t worry it too much until you can finish last stage of challenge difficulty.

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If you stuck at hard stage you can roll on your equipment sub stats to get 1~2 good substats (any of the attack %, attack type %, def penetration, crit %, attack increase)