Ah, that is a little advanced.
You see, because you defined the UFO object in the spell card script, it ceases to exist once the spell ends. To get around this problem, you will have to put the code that defines the object inside the stage script that the boss script is part of. To call the object at the right time, I advise creating a CommonData "UfoSpawn" that is set to false. Then you code the stage script so that it will spawn the object and set UfoSpawn to false if its value was true before. Then you make it so that, at the time when the object would be spawned in the player script, you set the UfoSpawn to true instead.
Understood?