Halo: Combat Evolved Anniversary

For this achievement it can be done in coop with the Bandana skull on for infinite ammo.

When you are back outside at the end of the level, after taking the lift up. You can just stay there and continuously kill the enemies that come towards you with the shotgun. They never run out since they continuously respawn, so happy hunting.

There is also a health pack there if the enemies damage you.
The shotgun is found on 2 enemies in the level as you are racing out and 1 is laying on a platform covered with dead enemies.

When you are racing out for the achievement which needs you to escape in under 21 minutes, when you do escape you will be in the area I am talking of. So you can easily get both achievements.

I did this and the 21 minute achievement on legendary in a single playthrough.

Very simple just progress through 343 Guilty Spark on at least Heroic and kill the Flood Human shaped guys, will pop after you have killed 50 in total you don't have to finish the level. For me i popped just before the lift outside suggesting there is enough before you leave and extra should you require it.

Shotgun seems to be very effective against them, i turned bandanna on so i didn't need to worry for ammo.

Just a bit of clarification; the Flood Combat Forms are the ones with legs who can carry weapons. If you kill every single one you find you'll have the required number killed long before the end of the level.