Hello friend, I downloaded your .capx and after a few tests I found out what is going on.
You see the problem here is the position of the object that you called "Barrier". If you set the Gordo HP for a value really low, or increase the player's attack, you'll manage to defeat the boss pretty early, destroying it while it's still in the middle of the screen.
When you set the player attack to 1 and the boss HP to 30, it take too long for the player to defeat the boss, and when you finally manage to defeat it, it's already too close to the end of the screen, and then the problem here is the boss animation "die" has a HUGE collision box, way bigger than the collision box of its other animations. So you manage to kill the boss and the "die" animation works normally, but since its collision box is so big it hits the "barrier" object and destroys the enemy before the animation happens.
To solve that you can either decrease the size of the "die" animation collision box, or push the barrier a little bit backwards so the boss has more space to die, or add a condition to only destroy the boss when it hits the barrier IF "die" isn't playing.
The game looks great by the way, keep up the good work ^^