Clowns, wave after wave of alien clowns. Headshots are the only way to kill them, as their weakness is their nose. They can introduce the cotton candy laser, the rubber ball grenades, and the invisible clown car. (Anyone who picked up on the Killer Klowns from Outer Space reference before this line gets +1)
Can they drink your blood through a crazy straw too?
Prometheans would beat the Flood as they are totally digital, and thus immune to conversion.
Didn't the Didact create the Prometheans while they were at war with the Flood? And didn't the Forerunners still get defeated? (Until they set off the Halo Array of course.) --- Everyone said it a was a miracle when Jesus hit that grand slam in the bottom of the 9th. I just thought it was a lucky shot.