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.
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.
Jotaku, Anime Game Gamer
I. Super Mecha Champions NA Legendary Tier
II. Tokyo Ghoul: re Call to Exist NA Top #2
III. Fate/EXTELLA Link: NA Top #9
IV. Senran Kagura PEACH BEACH SPLASH: NA Top #3
I. Super Mecha Champions NA Legendary Tier
II. Tokyo Ghoul: re Call to Exist NA Top #2
III. Fate/EXTELLA Link: NA Top #9
IV. Senran Kagura PEACH BEACH SPLASH: NA Top #3