Hey musty Im playing a combination of brig and mercy and lucio and it's a great way to level up right now. Im maining brig but then switching in some situations. Seems an easy win just picking the right support for the situation. here is a few tips off top of my head.
When attacking on levels with a choke like holywood,kingsrow,Eichenwalde you need lots of healing to push through so brig isn't good a choice. Attacking on these maps mercy is needed but brig is ok in defence.
When you play brig you wanna ask your team mates for rein zarya mercy.
Only play brig if you have good healing, if the other healer picks zen just go mercy instead.
Brigs worst match ups are against phara and doomfist so if the other team are playing either of these switch to lucio or mercy.
Also if other team is playing double sniper switch the brig to either lucio or mercy as there isn't much she can do.
If other team is rocking a tight rein zarya brig you don't wannt play brig, any time you go to bash they will just bubble or counter bash. I like Lucio aginst this.
If the other team has a brig and is out playing you just go lucio and boop her away anytime she comes near your team.
You positioning with brig should be relaxed and a few metres behind your tanks with your shield up and then just move in when you wanna shieldbash/mellee/whip combo then move back.
With mercy just find a decent hanzo or mcree and damage boost him for some quick kills.
Brig can destroy a lot of the characters 1v1 just be patient then push in when needed. You can practice in ffa to get your mellee skills up.
I did post a few videos of brig few pages back not sure if you see them. I can remap a record button and get some more if ya want.
https://forums.overclockers.co.uk/threads/overwatch.18635265/page-264
Also with brig always look at knocking them off the map. You can shield bash then side step then whip to send them whatever direction you want. There also places like the stairs next to point on numbani just throw out a long ddstance whip up the stairs and it launches people off the back of the map.