how to do coup de grace
-flash grande
-headshot then slide and melee
-quickshot then slide and melee
-Saving your partner while they're grabbed
-Behind the enemy, do the RT > RTcombo and you'll get the prompt.
So this is from the Mercs guide that was linked to, but I've been experimenting and found more. As I said before the key is to get the enemy to the end of the spin animation. So basically, you need to stun an enemy twice before they recover from the first stun to trigger the spin stun. Also to get the coup de grace you must either be facing the enemy's front or front side at the end of the animation when they are slumped/on their knee.
1 stage spin stun
-flash grenade
-reverse elbow
-save partner while grabbed (does the coup de grace)
-RT>RT combo from behind enemy
2 stage stun
-headshot then slide into feet or dash melee
-quickshot then slide into feet or dash melee
-shoulder shots to turn enemy then slide into feet or dash melee
-dash melee then slide into feet (this is one of the most effective and easy ways to do it)
-slide into feet twice in a row (easier said then done on the 2nd slide)
-dash melee twice in a row (not very useful because you need a slight run up)
-RT>RT combo from behind or side (can be done even when the enemy is already in a first stage stun, trick is to hit them with the 2nd attack so not to activate a heavy finisher)
the only thing to note is using a dash melee for the 2nd attack in 2 phase stun has a chance of killing the enemy outright rather then spinning them.
Damn it I love this game.