Resonance of Fate

This achievement is to complete the side quest "Rebel Termination order" in chapter 16. Beware that once you visit the guild bounty board in Ebel City, you have no other choice but to undertake this side quest as the location of the battle is on rainy bridge.

As stated by Nyren546, the elderly man can be a little difficult as his melee attacks do quite a bit of damage. That being said, if you do hero actions and aim to land far away from him, you can fill him up with scratch damage and he'll never get a chance to hit you. I completed the battle with only taking damage from his two flunkies.

He does recover scratch damage pretty quickly, though I was able to fill up his gauge with 3 attacks from my machine gunners with a full charge (Vashyron lvl 35 and zephyr lvl 34 machine gun).

The use of hollow point rounds and hi-polymer padding (30% physical damage resistance) are recommended for this battle.

This achievement is to defeat the Elderly Man which is a side quest in the last chapter (16). He can be somewhat difficult since he does enormous amounts of damage but only from melee swings. If you just focus on him and load him up with scrap damage using Hollow Point Round EX then it shouldn't be too hard to finish him off while only being attacked once or twice.