p.t. 3 mesopotamia
I am going to begin a series of posts where I explain why certain abilities from across fiction would be good counters to a flood invasion from the Halo universe.
First up we have: DECAY'S EVOLVED FORM from My Hero Academia
Decay belongs to one of MHA's main antagonists Tomura Shigaraki.
The base gives him the ability to completely disintegrate anything he touches with all five of his fingertips. While that doesn't sound all that threatening it is DECAY's evolved form that allows Shigaraki's DECAY to spread from one thing to another.
Picture it like this: Shigaraki touches the ground which causes his DECAY to spread to anything and everything making contact with the ground.
In canon this ability allowed him to destroy half a city in an incredibly short amount of time with barely any effort at all.
This makes it extremely suited to fight the flood because it doesn't require direct contact with them, can destroy hundreds if not thousands of them at once, turns them into dust which prevents the spread of spores and destroys the posibility of any left over matter from the contaminating other people or natural resources.
The ability also no longer requires all 5 fingertips to activate, though reducing the number of fingers reduces its speed and range.
The only mayor downside to this ability is that the user is not inmune to its effects.