How do you deal with Mercy in Platinum?

Ok so I've been Platinum every season. Got placed around 2400 this season and have since climbed to 2641. I play DPS but also play Winston a lot of the time, and that means a lot of the responsibility of dealing with the enemy Mercy falls on me. At Platinum, teams are coordinated enough to know that the Mercy needs to be taken out before she rezs, but that almost never actually happens. Mercy players have enough game sense at my level to be able to avoid everyone on my team pretty well by just flying from target to target, and despite everyone saying they're going to focus the Mercy, they usually don't. Now I'm not saying I'm not guilty of this. A lot of times in the heat of battle if I can't find Mercy I'll just focus on other low health/important targets, but when I do focus her, it's almost never enough. Now when any game can be swung in the enemy team's favor at the last second by a clutch Mercy rez, you can see why I want to have some more control over this aspect of my games, since she's in just about every match. It's extremely demoralizing to have a successful push where you blow a lot of ults, and then suddenly be forced to fight another 6v6 or worse without ults. Sometimes it feels like there's no way I could've wasted the time to go find the Mercy who's hiding behind a wall, as it would have left my team undermanned and wouldn't have been worth it. On the other hand, letting her get a rez off pretty much nullifies everything my team has already done, and since you basically need ults to initiate/win team fights, you can't just save them until after Mercy rezs. With Winston it's especially hard to kill her, because I often have to use my jump to get to her, and after doing a bit of damage to her she'll just fly out of my range way before my jump is off cooldown. I really feel like a lot of my games come down to which team's Mercy makes the better plays. Now I understand that theoretically, since I'm the only constant in every match, if I'm performing well consistently, I'll eventually climb, and I have been climbing, but ultimately killing the enemy Mercy at the right times would allow my games to not feel like they're completely decided by random chance. Anyway, does anyone here have any tips? Is it too much to hope for for people at my rank to consistently deal with enemy Mercys? Do I just need to communicate better? This is probably the one thing, aside from dive meta, that's really bothering me about competitive at the moment.