What causes Coup de Grace?

What causes Coup de Grace in Dragon Quest 9?

Solution 1:

This link explains what helps to improve the trigger of the Coup de Grâce (see Trigger table)

DragonQuest Wiki

The frequency of a Coup de Grâce occurring is amplified by the following conditions:

Critical fan equipped   +6%
Overcritical fan equipped   +7%
Hypercritical fan equipped  +8%
Dire critical fan equipped  +10%
Combat action medal     +3%
Martial artist, Ranger, or Luminary     +3%
1 other party member primed     +0.8%
2 other party members primed    +10%
3 other party members primed    +36%
10% damage taken    +0.75%
20% damage taken    +1.5%
30% damage taken    +3%
40% damage taken    +6%
50% damage taken    +12%
60% damage taken    +24%
70% damage taken    +48%
80% damage taken    +95%

Solution 2:

It also depends on what you're fighting. If it's any easy kill monster, your chances are really low, but if you're fighting a major boss (such as Corvus) your chances are increased.