coup de grace is a full round action while you can ready only a standard action, and she would have to be helpless anyway...
She is helpless. She's at -7 HP! (But, the PC's don't know that.)
I know about the coup de grace. I was just giving you an idea of the general plan. Who knows exactly how it will play out. As for his Ready Action, he's got a regular attack Readied, against a helpless victim, which means Stenna can't Parry or Dodge, effectively DEX 0, and is -4 to defense for being prone vs. melee attack. That makes her AC 3.
If he can't hit that, it's not in the cards for her to die at this point.
You should set an objective and give the PCs exp for meeting that objective.
Already done! In the Conan RPG, XP is completely story based. There is no formula for getting XP for killing monsters, using skills, or gaining treasure. It's all about the story. And, it's all in the hands of the GM.
What I decided was that I'd give a pretty good XP award for the goal of going after the sound of the ambush in the first place. The PC's could go back to the village first and try to get help. If they do that, no big bonus award for them, and they'll arrive too late. The ambush will be over. They'll just find the dead NPCs and be trying to piece together what happened.
That's one of my contingency plans.
If they chase him all the way back to his lair, they get another big XP award for finding the Grath cave.
If they rescue the little girl before she's sacrificed to the Grath god, they get more XP.