As briar, During the mid-game, all you need to do is look for picks all around the map. This is the time for lane swaps so it should be easy for you to get kills. 

Target immobile carries as much as you can. They can't do much to save themselves and you will be able to escape using your Ultimate R.

Look for flanks on the enemy team and see if you can catch them off-guard by using the fog of war. This will make team fights really unfair for the enemy team.
 As briar, During the middle of the game, you just have to look for picks all around the map. It's time for the track swaps so it should be easy for you to get killed. Still target carries as much as you can. They can't do much to save themselves and you'll be able to escape with the help of your Ultimate R. Look for the sides of the enemy team and see if you can catch them out of sight using the fog of war. This will make the team fights really unfair for the enemy team.