Hero natural attack x rune and leader bonuses to atk x 8 x (1+crit damage bonus) x (1+bonus boss dmg)
8 appears to be the natural crit multiplier
Enemies seem to have some form of def. This is probably a flat figure.
Leader skills seem to contribute additive to the atk bonus, but tag skills seem to multiply.
This might imply that the tag skills that read (+X amount added to team attack bonus) is simply adding additively to the character sheet. And it would explain why they're such huge figures since something like that is inherantly worse.
Because 3 rank 5 +15 runes will give at least +90% atk, all forms of atk bonus increases that are additive are generally less valuable. (Rare lines, leader skills) In other words, they scale poorly.