There are four main parameters that determine the amount of total damage D(sum) that your unit can deal to an opponent:

1. Your unit's base damage D(bas), expressed in hit points (HP)
2. The base damage modifier MD(bas), which depends on the comparative characteristic of your unit's Attack and the victim's Defense, expressed in hit points (HP)
3. The offense modifier M(of), which depends on the presence and level of the Offense secondary skill of your hero, expressed in hit points (HP)
4. The luck modifier M(luck), which manifests if Luck is triggered in the current combat round. (Note: we do not consider morale triggers, as that is simply an opportunity to repeat a turn in the current combat round and total damage calculation in that case is done separately).

All other modifiers, including all sorts of spells, hero and unit specializations, only modify one or more of the parameters listed above.

Let's consider these parameters in more detail.

1. Your unit's base damage.
Each unit has a certain range of damage values [Dmin;Dmax], within which the base damage for calculating total damage is randomly selected. The value is chosen randomly in each combat round.

Two spells can affect base damage:
* Bless – all units deal fixed base damage equal to Dmax [or Dmax+1 with advanced/expert magic]
* Curse - all units deal fixed base damage equal to Dmin [or 0.8*(Dmin-1) with advanced/expert magic]. However, the damage dealt by a unit cannot be less than 1 HP.

2. Base damage modifier.
If the attacker's attack and the victim's defense parameters are equal, the base damage modifier is zero. If the attacking unit's attack (including all artifacts) is greater than the victim's defense (Def), for each unit of difference, the modifier increases by 5% of the base damage value (the maximum excess allowed in the game is 300%; if the Attack-Defense delta continues to grow, the effect does not increase). If the attacking unit's attack (including all artifacts) is less than the victim's defense, for each unit of difference, the modifier increases (in absolute value) by 2.5% of the base damage value (maximum reduction - 70%), and in this case, the modifier has a negative sign. That is, it ultimately reduces the unit's total damage.

MD(bas) = (Att-Def) * 0.05 * D(bas)
(if attack > defense) maximum of 300%, meaning max damage occurs when attack exceeds defense by 60.

MD(bas) = (Att-Def) * 0.025 * D(bas)
(if attack < defense) maximum of 70%, meaning min damage occurs when defense exceeds attack by 28 or more points.
Note: From this, it is easy to show that the base damage modifier can take values in the range [-0.7; 3.0], where one unit is taken as the base damage D(bas). Or, in other words, the base damage adjusted by the modifier will ultimately be a value of [0.3; 4.0].

The following spells can affect the base damage modifier:
* Bloodlust – adds +3 (+6 at advanced) to attack for the specified (or all - at expert) stack
* Berserk - adds a single (one-and-a-half times at advanced magic level, double at expert) amount of defense points to the attack of the specified unit (the unit's defense is zeroed out)
* Executioner – adds +8 to attack against all units of 7th level and above
* Prayer – adds +2 (+4 at advanced) to attack, defense, and speed for the specified (or all - at expert) stack
* Precision - adds +3 (+6 at advanced) to attack for the specified shooter (or all shooters - at expert).
* Weakness – reduces attack by 3 (6 at advanced) points for the specified (or all - at expert) stack
* Stone Skin - adds +3 (+6 at advanced) to defense for the specified (or all - at expert) stack
* Disintegrate beam - reduces the desired unit's defense by 3 points (by 4 at advanced, by 5 at expert). This is the only spell mentioned whose effect lasts until the end of the battle or until Remove Spell is cast. Also, a feature of this spell is that it can be cast multiple times on one unit, and the effect will stack; thus, a unit's defense can be reduced to zero over several combat rounds.

Thus, it is important to understand that all the spells listed above affect only one of the parameters making up total damage, namely - the base damage modifier. These 8 influence spells do not affect any other total damage parameters.

And the effect of using these spells will depend on the significance of the base damage modifier for each specific unit. For example, if the attacker's attack is greater than the victim's defense, then the Bloodlust spell will increase total damage by 6*5%=30% (of the base damage value). The action of this spell for such a unit exactly corresponds to the expert Offense secondary skill. However, if the attacker's attack is significantly lower than the victim's defense, the effect of the spell will be twice as weak and will only be 6*2.5%=15% of the base damage value.

Based on the above, it is easy to trace the advantages and disadvantages of a particular spell in each specific case. For example, for units such as Spearman, Fairy, Harpy or Cerberus, which have a wide range of base damage - expert Bless (which will increase base damage at least twofold, and this will be reflected not just in one, but in all components of the sum!) will look much more effective than Bloodlust, which although it gives an apparent strength of +6 attack, will ultimately bring only 30% of the base damage for only one of the four terms.

3. Offense modifier
The third component of total damage - the offense modifier M(of) depends on the level of development of the Offense secondary skill (if it is present among the hero's skills) and amounts to 10-30% of the base damage D(bas). The base damage modifier is not used in the calculation of this parameter, meaning none of the spells mentioned above or any artifacts will change this component of total damage. It is strictly fixed and determined only by the actual random base damage in each combat round.
Special attention should be paid to heroes who have an Offense specialization (Crag Hack and Gundula). These heroes have a bonus that increases the offense modifier (and only it) by 5% for every level of experience reached.

M(of) = 0.30 * (1+0.05*Level) * D(bas)
specified for expert level Offense skill, expressed in hit points (HP)

It should be noted that these percentages are relative rather than absolute; i.e., a tenth-level Offense specialist with an expert Offense skill (bonus +30% base damage) will ultimately have M(of) = 30*(1+0.05*10 Level) = 45%, and not 30+5*10 = 80%, as is sometimes mistakenly calculated.

Since an offense specialist initially has the basic Offense skill, their bonus at the start is M(of) = 10*(1+0.05*1 Level)=10.5% of base damage. The maximum achievable level in the game is 108 Level, which would allow a specialist in this case to have a bonus M(of) = 30*(1+0.05*108 Level) = 192% of base damage.

The offense modifier is not used in the calculation of damage dealt by shooters at a distance.

4. Luck modifier
The simplest and most understandable modifier of all four. Because if luck triggers during an attack, the unit deals additional damage equal to the base damage D(bas) determined for that combat round. Obviously, if a Bless spell is cast on the unit, for example, the luck damage will be (Dmax+1).
The only known feature of this modifier is the fact that when calculating damage from a unit that can attack several enemy stacks at once (Hydras, Psychic Elementals), luck applies only to the unit being actually attacked. The others receive standard damage without considering the luck modifier.

M(luck)=D(bas)

5. Total damage modifiers

5.1. Spells

It is necessary to separately mention the three remaining spells that modify the total damage dealt to a unit.

* Shield – reduces by 15% (30% - at advanced) the total damage dealt to a unit in melee combat. At expert - protects all stacks in the hero's army.
* Air Shield - reduces by 25% (50% - at advanced) the total damage dealt to a unit from ranged attacks. At expert - protects all stacks in the hero's army.
* Forgetfulness - reduces final damage dealt by the specified enemy shooting stack by 50% (at basic water magic skill); at advanced, this stack (and at expert - all enemy shooting unit stacks) loses the ability to shoot until the spell expires. The unit's ability to attack in melee is preserved.

The first of the two proposed spells is often paired with the Stone Skin spell, and probably every player has asked themselves which of these two spells is more effective. Well, my opinion is that if there is a choice between Stone Skin and Shield, Shield will be more effective because Stone Skin reduces only the attacker's base damage modifier by 15-30% (one of the components of total damage). At the same time, Shield removes 30% of the entire total damage value (which is always more effective).

The effect of the second of the two proposed spells is essentially similar to the first; however, there is one significant bug that must be considered when using Air Shield – if this spell is cast on a unit, instead of half damage, it receives double damage from shooting towers during a city siege. This can significantly complicate the capture process if you are unaware of this nuance.
5.2. Armor secondary skill

The Armor secondary skill possessed by the hero being attacked affects the total damage dealt to their units and reduces it by 5-15% depending on the level of development of this skill. To calculate the defense modifier M(def) for specialists in the Armor secondary skill (Mephala, Nile, Tazar), we use the same calculation mechanism as for Offense specialists - for each level of experience reached, the secondary skill bonus increases by 5% (relative).

Also, I think it should not be disputed that an expert in the Armor skill will in most cases reflect a greater number of attack hit points than an expert Offense secondary skill would bring to the same attacker, even considering that in relative terms (15% vs 30% for Offense) it looks less impressive. It is applied last in the calculation and complements the effect of the Shield or Air Shield spell.

M(def) = 0.15 * (1+0.05*Level)
specified for expert level Armor skill, expressed in %.

In total, total damage will be calculated as the sum of the specified parameters, adjusted by the defense modifier (the effect of spells is not reflected in the formula, but is implied):

D(sum) = [D(bas)+MD(bas)+M(of)+M(luck)] * [1-M(def)]