Simplest answer is focus fire, especially on the Pharah. If someone on your team is playing a hero that can reasonably hit a Pharah in the air, they should probably be shooting at her.
The whole reason the dive comp got phased out of the last meta was the strength of Roadhog as he can punish anyone even slightly out of position. Dealing with him is easier if you are in a death ball lineup where your team is all huddled behind the Rein shield. In a lot of situations he should also be priority number 1 to take out, so communicate with your team to take him out as quickly as possible.
Counter Mei by running dive comps and not constantly trying to push through a choke, where Mei typically shines. A general rule is that after she uses her wall, your team charges her like madmen as she becomes significantly weaker at that moment.
This stuff can get really complicated if they are running multiple of these types of heroes that really dictate a team's playstyle, but generally the team will be weaker if they have to sustain two or more heroes in that vein.
Hopefully this helps out both you and any other players here that struggle against those heroes. For the record I am currently jumping between Diamond and Masters on console, so while I am by no means a god at the game or anything, I feel like I got a pretty good feel for team comps and countering.