[Jotaku]

In short, having higher "defense" and "skill damage reduction" will decrease incoming damage from enemy's "Skill Amplication".

However, no matter how much "defense" point you have, it will not protect you from enemy's "extra skill damage" ("extra skill damage" is something like true damage or flat damage). What protect you from "extra skill damage" is "skill damage reduction". Checkt the formula at https://eternalreturn.fandom.com/wiki/Formulas

Skill damage reduction directly reduces extra skill damage, and because extra skill damage occurs per instance it doesn't take much esdr to significantly reduce esd output. That's why they reverted mith chest. If you don't have any esd, esdr will still take a flat amount away from your base*ap%*amp

Recap
1. Both "defense" and "skill damage reduction" can help you from enemy's "Skill Amplication".
2. Only "skill damage reduction" can help you from enemy's "extra skill damage".
3. Usually Combination of "Skill amplication" and "Extra Skill Damage" can make better result in terms of DPS than having only one of them in your build.