Just some thoughts and ideas of the Firefight mode in Reach (if there is going to be one).
Okay, of course the round starts off and a Phantom drops off let's say some Grunts, Shielded Jackals, and maybe a few Minor Domo Elites to get things started.
Once that wave is done with then send in some more Grunts, Jackals (shielded or not, I don't really care), and some more Elites (low ranking ones of course). And once you're done with that round...
Zealots with Energy Sword to compete with the Brute Chieftans? Three of them would drop down along with jetpack Elites, like the ones we saw in the Halo 2 level, Cairo Station.
The waves and rounds would continue on with Grunts, Jackals, Elites, and maybe a pair Hunters.
Once we've hit the end of the rounds then three Ultra Elites would drop down with an Energy Sword or a Fuel Rod Gun.
If you survive to 7 sets (yes, it's a seven reference and you could survive to that long since you are a Spartan) then a Scarab would climb into the arena and blast things up until you are able to take out its legs and destroy the power core thingy inside of it, or it will stay for let's say 5 minutes then leave.
The game would then continue on like that. On the 14th set (yeah, you probably won't survive that long) you'd get another Scarab and on the 21st set you'd get another and so on.
Good idea? Just some thoughts. =D