For that you need to get a modded Sangheili vs Covenant gametype or Sangheili vs Sangheili Ai gametype and this is how:

1.- In the MCC main menu click in the profile icon that is at the top aside of your nameplate
2.- Click on your nameplate
3.- Click on "Find player"
4.- Type "TwistedFl3g" and click search
5.- Click "File Share"
6.- Select the gametypes
7.- Select the wanted gametypes and click on "Download"

Once you are in the modded firefight map selection in the choose game type new gametype variants it should be available at the end of the list