It could also be that the things that you see are simply part of the Magic spell, it's just an illusion that the world is destroyed or w/e, but the results unto the enemy are real.
Also, I don't think Lavos' eruption wouldn't matter. Even if his Destruction Rains From the Heavens were to touch down on his shell...For one, we know that Lavos survives his impact onto the planet in the first place, which would be expressed in tons and tons of energy right there. Secondly, based solely on Lavos' Spawn, we know that their Shells are nigh-indestructable, wouldn't it then be plausible to say that Lavos, being fully grown and fully powered, wouldn't bat an eye at his laser attacks that hit his shell? I mean, heck, Crono & Co. don't even bother trying to target the shell, yeah? I think there's your reason...