This table is always subjected to change.
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Below 30% Critical Rate, use Fatal Atk%/Atk%/Atk%.
Between 30% and 45% Critical Rate, use Rage Atk%/Atk%/Atk%.
Between 46% and 58% Critical Rate, use Fatal Atk%/Crit.D/Atk%.
59% and above, use Rage Atk%/Crit.D/Atk%.
This means if you spot a unit that has Slot (4) Atk% and is meant for damage then it can be replaced with CritDmg% (with the exception of units that use bombs)
Same goes with Fatal or Rage
It can also apply to units that use def% to increase their attack (E.g. Lizardman)
Credits to ApertureBear for providing this. [Click the name for the math behind this]
Before you complain about why I didn't use Rage+Blade (Atk%/CritRate%/Atk%) please note that you are sacrificing a lot of potential damage from the 4th slot just to bump up your critRate and have an additional 30% critDmg. Are you willing to sacrifice a whole 64% atk power just for an additional 30% CritDmg?
All Acc% and Res% slot(6) stats can be replaced with the primary Atk%/Def%/HP% if you are able to gather enough substats. With units that have no indication of Atk%/Def%/HP% (E.g. (Spd/CritRate%/Acc%)) refer to the type of unit it will be (e.g. Attack, Defense, HP[with support units, it's recommended that you find the highest above average using this to guide you.
Despair+Focus or CritDmg for Dark Marital Cat if against Light Bearman in Arena
Fire Death Knight should only use Violent if he has high enough HP stats. The same applies to Dark Sylphid and Dark Pioneer
All units will be following the fire format since that element was the first element I started with (meaning the E.Priest will be located in the 4* section