Hi! I've been trying to find something official on this or some datamined formula, but no luck. So far I've only seen some consistency on these numbers:

• Critical: x1.275 damage

• Effective: x1.25 damage

• Uneffective: x0.75 damage

• There's some random variation on the damage that can be done normally. I've seen around 10% more or less

But I can't confirm those 100% either. I played around with the numbers (level, level difference between Nexomon, rarity, Pow of the skill, ATK/DEF stats, STAB) with no luck. I couldn't find anything either by using them as percentages. It might be a complex formula that has its own constant number added to make it hard to decipher.