This mod changes some of the firefight waves to include Elite Zealots and the imposing Field Marshal. Sadly, squads cannot be created from scratch, so existing ones must be edited. This means the best candidate to become the Field Marshal is a Brute Chieftain, and by removing them from the roster it severely reduced the Brute presence in Firefight, so I decided to replace all Brute waves with Elite-based ones instead. 

Elite General waves were the best candidate to become Zealots, and so to keep Generals as an option I replaced Brute Captains with them.
So, as a guide; any waves that in default Reach contained Elite Generals now feature Elite Zealots in their stead, and any waves that featured Brute Captains now feature Elite Generals. The Field Marshal replaces the 'Brute Chieftains' wave, and there are some caveats to keep in mind when using this wave.
It also includes an entirely new weapon used exclusively by the Field Marshal called the Plasma Onager. It is essentially a Plasma Rifle with a slower rate of fire that shoots Phantom turret rounds.

IMPORTANT!
When using the new 'Brute Chieftains' wave, it contains a single Field Marshal and a single Elite Zealot. This means that when they spawn with dropships disabled, the next wave will IMMEDIATELY be called to spawn also, because the wave is below the minimum threshold for wave completion (I believe the minimum is four enemies). Keep this in mind! A nice combo is to have the Field Marshal ('Brute Chieftains') wave followed by a Zealot ('Elite Generals') wave to give a full complement of Zealots similar to the final encounter in campaign.
However, if this wave has dropships enabled, the next wave will not be called until after a set amount of time, which seems to be around 60 seconds. While the Field Marshal is a formidable foe (the toughest Elite by body vitality and shield strength, with a weapon that can three-shot kill you on normal if it connects), I feel that the best experience is to spawn him without a dropship so he gets the next wave as backup. On Legendary however, you might want to face him and his single escorting Zealot alone.
below is the full list of edited waves and what they now contain.

WAVES
Brute Chieftains -> A command element composed of one Elite Field Marshal escorted by a Zealot.
Brute Kill Team -> A mixed force of Grunts, Elite Ultras and Elite Generals led by a Zealot.
Brutes -> A kill team of Elite Ultras led by Generals.
Brute Patrol -> Grunts led by Elite Generals.
Brute Infantry -> Jackals led by Elite Generals.
Brute Tactical -> Skirmishers led by Elite Generals.
Elite Generals -> a strike team of six Zealots.
Elite Strike Team -> A mixed force including Skirmishers and Elite Rangers, led by Zealots.
                                                                                      
Elite Generals can spawn with the following weapons: Plasma Rifle, Needler, Plasma Repeater or Concussion Rifle.
Elite Zealots can spawn with the following weapons: Plasma Rifle, Needle Rifle, Concussion Rifle, Fuel Rod Cannon. They will switch to an Energy Sword if their shields are broken, as will the Field Marshal.

HOW TO INSTALL
First, navigate to your Halo Reach Maps folder, and create a backup of 'ff50_park.map' known in-game as Beachhead. Then simply copy the modded map into the maps folder and overwrite the existing one. Make sure you launch the game with easy anti-cheat disabled, and remember to replace the vanilla Beachhead map before playing firefight on official servers with anti-cheat on!