This trophy is awarded the first time you kill a "Mercenary" or boss titan with a melee execution. 

To initiate a melee execution, you will need to get a boss titan down to their third health bar. That is, you will have to destroy their shields and all of their bars of armour so that the final health bar, which appears as a yellow and black striped bar (sort of like hazard tape) appears. Then, before their shield can recharge, walk up to them and initiate a melee attack with . This should trigger an animated, execution-style animation, killing the boss and ending the fight.

The only mercenary you can't do this to is Viper, as his fight ends with a semi-cinematic set piece rather than a standard fight to the death.