Have a trigger outside the path that sets a global int to keep track if your player is off track or not. This will also allow you to change the same global int when the player exits the out of track area/enters the in track area.
Have the enemy comparing the global int to see when it switches. This will then set it in motion to start the attack sequence. Have the sequence watching the same int so when the player goes back on track it diverts the action (to cancel it).
The exact details of this will very much depend on how your enemy works and it's functions. Eg; Does it *poof* appear and disappear, does it start in a static location, does it need animation or will a simply tween work, etc.