To clarify before anyone asks questions, Millenniummon isn't dead quite yet. Parasimon's attack is powerful, but won't outright kill it due to high resistance to this kind of attack (basically ridiculously high special defense, if you want me to liken this to Pokemon).
That said, if anyone else wants to fire some kind of hugeass lazor/wyvern nuke/etc etc at Millenniummon whilst he's getting hit with the Omega Burst, it'll overwhelm it enough that the resistance doesn't matter. So if anyone wants to get in one last effective attack on Millenniummon, please do feel free to go for it.