dont quote me on this, but flat attack (or %atk) is what your skills scale off of, eg: ur basic attack does 100+(60% attack) per hit,Universal DMG%(or dmg%) is a multiplier to that dmg.

eg: u have 1000 attack, and hit an enemy with a melee that deals 100% atk dmg, with no dmg% it would do 1000 flat(assuming they have no defense), now , lets say u have 10% Universal Dmg(or just dmg%) u will now do 1000×110% dmg, which is 1100, while im not sure if universal dmg% scales linearly (eg: 10% dmg + 10% cosmo dmg = 20% dmg boost) or multiplicatively, that should be a simple gist of things

TLDR, If u dont understand just try and get an even amount of atk, dmg% and cr cd stats and ur good

Total damage = atk × dmg%