Mega Brawls are special Brawls that allow you to take part in epic conflicts against more powerful adversaries. Whilst in many ways these function similarly to regular Brawls, there are certain key differences.
When active, notice a timer bar in the upper left of the Brawl screen. This shows how long the event will be available. The objective is for players to collectively score points against the Mega Brawl as possible before the timer expire, by engaging it and achieving victories.
Fighting in a Mega Brawl is no different in nature from fighting in any other Brawl. however, note that a mega Brawl cannot be engaged with AI teammates. You must team up with other players. Also, it cannot be initiated with fewer than the maximum number of participants. As with conventional Brawls, Mega Brawls may be engaged on Normal or Crisis difficulty. The Selected difficulty determines how many points will be earned in the event of a victory: 6 for Normal, 25 for Crisis. Losses in a Mega Brawl yield 1 point, regardless of difficulty.
The Scoreboard in the upper right of the brawl selection screen shows how many points you've earned in total, along with the names and scores of the five players who've earned the most against the current Mega Brawl.
There's also a separate value entitled 'Community Points' to represent how well the playerbase has done as a whole. Once the timer reaches zero, new mega Brawls can't be engaged.
The "start a Mega Brawl!" button will instead be replaced with a button labeled 'View Results and Retrive Loot'. Clicking this button will reward players with loot, based on their personal points tally in conjunction with that of the community.
Details about a mega Brawl's rewards can be seen by clicking the "View Loot" button above score table on the selection page.