Here's a video guide for those who would rather watch somebody do it.
KeyesPar Score: 20,000
Difficulty: Legendary
Skulls: Boom, Pinata, Grunt Birthday Party
Level time: 26:32
Level score: 58,704
NOTE: If you die, quickly press Start and Save and Quit. Then press resume and you will save time and you can make a no death run getting you an even higher score. You must Save & Quit before you respawn again.
Tips:- When farming kills on the exploding flood, use pistol ammo but also melee the popcorn that come after you for infinite grenades. If you go to a high enough score, then you probably won't need the Save & Quit trick as you will still have a lot of score remaining at the end of the level.
- Make use of your grenades
- Go for headshots
- If you're using the Save & Quit trick, then just try speedrunning the rest of the level once you reach your Par Score.
- Try completing the level as fast as possible.
- Have patience. The video makes it look easier than it really is.
Video Credit: Halo Completionist (Youtube Channel)