Titanfall 2 – Coup de Grace Trophy / Achievement Guide

Titanfall 2 – Coup de Grace Trophy / Achievement Guide – Destroy a Mercenary Titan with a melee execution in the Campaign [Bronze / 15G]

Mission 9: The Fold Weapon (Slone Boss Fight)
You can do this against the final boss called “Slone”. This doesn’t work on normal titans.
Damage Slone until her health is nearly depleted. Then run up to her and perform a melee attack (when the yellow health bar starts to show). This will result in a special takedown animation.

It can only be done against certain bosses. On some bosses it doesn’t work because they explode instantly and you can’t melee them.