The best way to farm XP in Mythic Firefight is to use the Legendary Warzone XP Booster which you can find by googling “Halo 5 Warzone Legendary Booster”
I don’t condone doing this, it is just an observation I made in earning XP.
It is best to be in a group so you don’t impact anyone’s game experience, but what you would do is win the first round and throw the second round. you will earn just about 15,000 XP or a little more, with games averaging about 5 minutes. so the math will come out to 15,000 XP x 12 (games that take 5 mins) = 180,000 XP an hour or 5.5 hours for 1 Million XP. Normally it would take about 11 hours with legendary boosters to hit 1 Million XP, the method above would half the time to earn the XP.
This method is super fast compared to just playing the game and it would still take you almost 300 hours. Mental. I think I’ve played the game for about 70-80 hours, and I believe I am not even a 1/10th of the way there.
I wouldn’t view this grind to be that crazy. The only thing that needs to be done to reach the final rank is just to play approximately 31 days of Mythic Warzone Firefight (w. XP Jackpot or Rare XP Boost) or double the amount of time in the Heroic equivalent which isn’t that bad and pretty standard in terms of time ivestment requirements for the final rank.
I have currently managed to reach the final rank in the follow Halo games and are debatting if it’s worth to do the same thing on MCC.