Brawls are 2v2 battles in the PvP section. this requires you to win the 2v2 in actual online matches and not against the AI.
Best way to get this is with a friend, of course, this is not always possible and matchmaking might take a while if you don't have a partner who can also slightly carry himself.
The community is quite nice with gentlemen's rules, like not ganging on a single opponent and not ledging (throwing you off the map) and if people are nice enough to follow that I really advice on you and your buddy following those rules as well, if you don't it will likely bite you in the behind.
In the end it comes down to learning your character and keep trying till you get there. 1v1's will likely train you and your character the best, but in a 2v2 you might have the luck of your partner being able to defeat that one character you struggle against.
Last advice, when trying to find a friend, find someone who doesn't main your character as you can have every Hero only once on your brawl team.
Good luck, keep training and you will eventually get this.