On the mission Blue Team there is a part where there Covenant keep dropping, and never end. I like to play this for Firefight sometimes. It is the part with Banshees and you have to blow up those exhaust/cooling pipes. There is also where you can skyjack a Banshee Ultra.
Same with the mission on Reach where you do the space battle. If you sit on the beach and don’t go in with the rest of noble team enemies keep dropping and so do marines. I played this with my brother for countless hours. One of my fondest Halo memories.