The Flood are not a mere parasite, they are a psychic gestalt consciousness that has existed for over 100billion years, living and dying at whim and resurrecting themselves as they see fit in what form they see fit. They are the latest incarnation of the Precursors, and with that comes the ability to warp space and time, destroy planets with contemptuous ease, and slice star systems apart using Star Roads, invincible constructs born of the Precursors psychic might. Their logic plague can infect AI thousands to millions of times more advanced than the Droids of SW. And The original 12 Halos can fire in linear directions, they can park them outside of the galaxy and just shoot it, which is what Omega Halo did to the Large Magellanic Cloud.