Most people use their main heroes to do logistics work or convert to Siege hero.
However, with Brother-in-arms, your main hero is now a very powerful combat hero with crazy Area of Effect ability. Please start converting your main hero into a fighting machine. Rebirth and reassign stats to Leadership or Power, Enhance stats to increase Leadership or Power, give equipment to boost Leadership/Power, give ability for combat, passive abilities are preferred.

thanks to PigFace for the image

The Brother-in-arms ability is similar to Pang Tong Thunder Fury, and it can actually be much stronger if you organize with your brother well.
As described, the number of brothers from the same garden on the battle field affects the number of targets.
To maximize this ability, create a mailing list of all your brothers (in the garden).
Coordinate and place all the main heroes of the 10 brothers in the same city. The stats of individual hero may not be great, but together, the effect will be fearsome.
It will be like 10 Pang Tongs that hits 10 targets when the ability triggers.
For Brother in Arms skills, here is what I know so far (correct me if I am wrong):
1. Damage: Same like singular focus :  130%+ (10% x skill level)
Note:some source say is -10% less so it’s 120%+ (10% x skill level)
2. Trigger Rate: 40% Chance
3. AOE : 3 + (number of brothers in battle / 3) == so if all 9 brothers in battle AOE will hit 6 enemies (maximum)
Requirement:  when you main hero gain more than 40 loyalty (so 41+), the skill singular focus will change to brother in arm after server maintenance at 4am.

Troop type: all type of troop
Gear effect: every 1% gear effect is about 4.5% in damage
Skill level: every level gain additional 10% damage
Charge attack bar: 2000

Additional note from MingLi about the Tavern : some peoples say that after loyalty = 40 you need at least 5 members to greet each day or loyalty will decrease faster than it increase.

Special thanks to MingLi, Astute and Jitkilan (L10) for help on this guide