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Don't confuse the numbers listed in a unit's stats with the concept of "base damage," which is inherent in the Luck action.
I believe that in each round and during each turn, a "damage modifier" is calculated randomly, but within the range of numbers specified in the stats... and an adjustment is made for attack/defense superiority...
In general, if it's not clear, read the Physics Guide:
1. Your unit's base damage.
Each unit has a specific damage range [Dmin; Dmax], within which the base damage will be randomly selected for calculating the total damage. The value is chosen randomly in each round of combat.
2. Base damage modifier.
If the attack parameters of the attacker and the defense of the target are equal, the base damage modifier is zero. If the attack (Att) of the attacking unit (taking into account all artifacts) is greater than the defense (Def) of the target, for each unit of difference, the modifier increases by 5% of the base damage (the maximum increase allowed in the game is 300%; further growth of the Attack-Defense delta does not increase the effect). If the attack of the attacking unit (taking into account all artifacts) is less than the defense of the target, for each unit of difference, the modifier increases (in absolute value) by 2.5% of the base damage (the maximum decrease is 70%), and in this case, the modifier has a negative sign. That is, in the end, it reduces the unit's total damage.
4. Luck modifier.
If luck triggers during an attack, the unit deals additional damage equal to the base damage D(bas), determined for that round of combat.
P.S. If you interpret luck as a bonus in the form of numbers listed in the stats of the attacking monster, then it becomes obvious that it is beneficial to take luck:
a) if you have weak attack parameters (almost all mages + Bog Fiend)
b) if your opponent has high defense parameters (Bog Fiend, Ranger, maybe Knight)