The problem with her arm ghosting is not from ambient light falling on her. Your flash doesn't need to be more powerful.Not quite true
It is from her arm moving during the long exposure and allowing ambient light to burn through the flashed area because the arm is no longer block that light and create a silhouette (which is then filled in by flash).
It doesn't matter if you use first or second curtain sync (or even hold the flash in your hand and manually trigger it by the test button), you do not want to keep her moving. You want to set her up in a pose that she can easily hold without swaying or moving for the duration of your exposure.
If she remains perfectly still she blocks the light just like when you create a silhouette. That will allow the flash to fill her in without ghosting.
Too much ambient light on the model will cause this too
My rules for doing this
1 make sure the model in as little ambient light as possible a good way to check is to take a shot with no flash ey should pretty much be a complete siloette
2 try keep the exposure to 5 seconds or less 10 is too long its too hard on the model I've found 5 is really the limit especially if you are takimg quite a few shots
3 camera on tripod remote trigger and 2 second timer I run out with the flash and point it where I want while the timer is going
4 make sure your expose balance is right that shot has too much flash power and being direct didn't help probably needed to be -2\3 to -1 flash ec on it