Shared damage
I just tested this with Dragon attacks and an attack all hero.
Battle item single target
Harpoon normally does 750 to target enemy. So 750 to Boss plus 0x 2x mobs. Total 750 damage.
With shared damage, 1/3 of 750 ( 250 damage ) is shared from Boss hit.
So with single battle items ( that ignore defense, defense buffs and defense debuffs ) each takes 3x 250 . Total 750 damage.
Battle item multiple target
Dragon attack normally does 400 to each enemy. So 1x 400 to each of Boss plus 2x mobs. Total 1,200 damage.
With shared damage, 1/3 of 400 ( 133 damage ) is shared with first mob hit, then a second time when Boss hit, then a third time when last mob is hit.
So with battle items ( that ignore defense, defense buffs and defense debuffs ) each takes 3x 133 which is 399 ( there is rounding ). Total 1,197 damage.
Heroes
With hero special skills, the same thing happens but this time the damage is adjusted based on defense, defense buff, and defense debuff.
This is very important for enemies like green 4* Kashhrek protecting nearby allies with red defense +94% buff against an attack from red 4* Colen.
This causes a lot of confusion because the entire damage equation is not known. But we do know it is non linear.
(Damage Calculation - #13 by SolemnWolf)
Defense debuff
With the above situation, red 4* Colen would tend to do more total damage because Wilbur also gives enemies a -44% defense debuff.
This means enemies take more damage, and that increased damage is shared.
In the above example battle items ignore defense, defense buffs and defense debuffs.
Harpoons, and Dragon attacks, do the same damage if the enemy has -44% defense or +63% defense.
See also