First up, you need to make two objects.

1: A spike or something.
2: Anything really

Put these off screen.

On object collision (Object: your obj 2)
Activate trigger (Trigger: your chioce) -- Triggers when boss hp runs out

Add an object.
Logic:
When shot
Activate trigger (your choice)

you get the idea
On trigger (that trigger that triggers when you shoot the boss)
Move set distance (Distance: usually 12 is what i use) (Speed: usually just max it out) (angle: 0)

now all you gotta do is make the death trigger do something (duh)