i like the last idea a lot (the shockwave one), im just not sure how to implement it, maybe it could create an "invisible" tiny sprite in the center of the crater, and the boss jumps up and coincidently lands on it when he causes the shockwave, then when he hits it, it creates a very long "invisible" sprite across the ground, and if you are on the ground (touching it), you recieve damage. ill have to test that idea a bit later, but thats great.
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oh, and 1 more question, would there be a way to make the screen shake slightly for like a sec or 2, like maybe have it activate similarly to the way the multiple hitpoint enemies do, where the script is activated by a function called up by the collision