1.0 Forward
A Coup de Grace is a super move. In this forward, I'll call Coup de Graces "CG". CGs save your day. Your fighting (Last Boss) and your health is pretty low. Look! Your Priest just got a Coup de Grace! Everyone is back to normal. To select your CG, when it's activated hit the Blue "Coup de Grace" button under "Items".

1.1~EFFECTS~
Warrior - Critical Claim (Guarantees a critical hit)
Mage - 0 Zone (No MP lost for a couple of turns)
Armamentalist - Voice of Experience (Random chance to earn more experience)
Gladiator - Tension Boost (Raises up the tension for the user to at least 50!)
Luminary - Disco Tech (Raises up the tension a bit and gets enemies to dance!)
Martial Artist - Roaring Tirade (Raises up the tension of the user and gets enemies into inaction)
Minstrel - Rough n' Tumble (Increases Evasion and counter attacking chance)
Paladin - Knight Watch (Makes the user invincible for a couple of turns. Also angers enemies sometimes)
Priest - Choir of Angels (It cures 40% of a character's total health and cures status effects.)
Ranger - Brownie Boost (Raises attack, defense, and breath resistance.)
Sage - Spelly Breath (Restores MP)
Thief - Itemised Kill (Guarantees an item stolen/dropped after battle)

1.2~TIME LEFT~
Yes, there IS a time limit for using Coup de Graces. Here is how many turns your Coup de Grace lasts for your level.
1-9 5 Turns
10-24 6 Turns
25-49 7 Turns
50-74 8 Turns
75-99 9 Turns
I haven't tested it out yet but I am 75% sure that revocating doesn't change the number of turns your Coup de Grace lasts.

1.3~GETTING A COUP DE GRACE~
Yes, you can change the percentages of getting a Coup de Grace! Woodus has it in the Dragon Quest IX part of here. Check it out! Weapons, ready members, and vocations all effect your chances of getting a Coup de Graces.

1.4~CO-OP DE GRACE~
If each of your party members uses a Coup de Grace on the same turn you score a Co-op de Grace!

1.5~CO-OP DE GRACE EFFECTS~
You could use these Co-op de Graces if you have the right vocation in your party.
Cast Away - Mage/Sage - Lowers MP cost to 0.

Electro Light - Minstrel/Luminary - Turns all enemies into metal monsters.

Haulellujah - Armamentalist/Thief - Chance to earn extra exp., gold, and items.

Omnipotence - Martial Artist/Gladiator - Raises everyone's tension 3 fold.

Soul Asylum - Paladin/Priest - Makes everyone invincible.

Quadraslash - Warrior/Ranger - All 4 party members inflict considerable damage.