Thank you to allenwfc for the original English translation and analysis. Thank you to Laif, Rai, Reed, and deliagwath for their contributions to the original English post.
Final Damage = Base Damage × Major Modifiers × Element Bonus Damage × Charge Damage × Weakpoint Damage x Damage Taken Debuff
Base Damage = ((Character Base Attack × (1 + Attack Buff Sum)) – Enemy Defense Power) × Weapon or Skill Multiplier
Major Modifiers = 1 + (Crit Damage (0.5 Base (50%))) + Core Hit Bonus 1 (100%) + Range Bonus 0.3 (30%) + Full Burst Bonus 0.5 (50%)
Element Bonus Damage = 0.1 (10%)
Charge Damage = 2.5 (250%) or 3.5 (350%), depending on character
Weakpoint Damage = 1 + (Boss Part Damage (Depends on Boss) + Other Damage Enhancement (Helm))
Damage Taken Debuff = Novel, Isabel
What Does This Mean?
From the formula you can see that crit damage is additive, instead of multiplicative in its effect. Since attack buffs are multiplicative, crit damage and crit chance are much less effective than attack buffs.
You can also see that enemy defense subtracts from your attack. This means that the closer enemy defense is to your base attack (read: the further you are in CP deficit), the more effective defense down becomes. However, currently enemy defense does not reach high enough levels for defense down to be as valuable as attack buffs.
Helm, Novel, and Isabel have unique buffs/debuffs that apply at the end of the damage formula. Their skill types are the most valuable.
Other than Helm, Novel, and Isabel’s skills, attack buffs are by far the best damage multipliers in the game, as they apply multiplicatively.