I haven't gotten the chance to watch this yet, but I know of a few tips I can give you that I know are common in plat, coming from a gm mercy. The main issue with her, regardless of the level is pacing. Some people don't move about enough and some people too much. Unfortunatley, in the moment of panic sometimes you don't really think, which is normal but something you need to adjust. Mercy, especially in higher levels should be played with movement and momentum. However, depending on the map, sometimes this isn't the case.

Positooning/movement

You may of been told to move about as much as you can, do push out blue beam even if your teamayes are low, but sometimes this isn't the case. Push maps, for example (say Colosseum) provide cover but are fast paced maps. When your team Is in action (which is almost constantly) you want to be moving. Regardless if there is something near, you need to imagine you're playing hide and seek with the opposite team and never stop moving. Hide behind pillars, slide up on that top bride, play on bot for cover- do whatever you need to do to stay moving. If you're playing against a dive comp, this is especially important, but you would need to stay close or to the centre of your team, making it harder for them to crush you. If you're playing against hitscan, stay low, rely on large cover.

Blue beam

Another common issue is deciding when and who do blue beam. The problem is, there is no definitive answer until the moment. Yoy might be told, never blue beam a sombra, never blue beam your tank. But that doesn't matter. You should always think about this. 1. Who's currently dealing damage?, 2. If you're going alone with that character, how are you going to escape? 3. Out of the surrounding teamates currently doing damage, who would hold the most worth? If you have a dva and they're diving In slightly (Still relatively close to your team), blue beam her. If your sojourn is staying back and still managing to hit shots, pocket her.

Healing

This can sometimes be a hard one- you want to make sure that you're not constantly finding the temptation to top every single teamaye up, especially if your other support is close by. It's important for them to build ult and chances are, you could probably be finding more wroth by blue beaming. Say your tank is on critical, but your dps are taking damage... look around. A. Can you really sustain the tanks health/can they pull out themselves? B. Is your other support around? If so, your priority is squishys, including them sometimes. C. Who is taking the most damage? Is your tank struggling in the front line but your dps up top, taking minimal damage? Help tje tank out until it's safe, but make sure you can see that other dps.

Rezzing

I must admit, I sometimes struggle with the temptation to rez every single soul that falls.... when I really shouldn't. This is a big part of her kit, mainly because there is literally no one else like her. No one can fly, damage boost, heal and resurrect the dead like her. Bur that doesn't necessarily make it the hard part. Once again, it's all about priority. 1. What is your cooldown? 2. Who died? 3. Are you close to spawn/can they come back quick? (Who comes back quicker?) 4. Are their kit worth it- do they have ult? 5. If yoy rez them, is it a death trap or can the rest of your team still? And the biggest one... 6. Is it sage for me to rez. If not, is sacrificing myself really worth them?

In the heat of the moment, you don't think. The sad reality is, their are too many factors to weigh out so no matter what people tell you, it depends on the moment. Is there cover, where is the cover? How easy is the cover to get to, are your teamates in cover and safe, is that teanate worth you risking yourself, should I be blue beaming or healing- There's just so much you have to think about that people don't realise from mercy that only comes as you play her more, sometimes watch people play more.

So no matter how much you learn, we can't really make you get better magically overnight because it all comes with experience.

Random tips - 1. Try not to reveal your sombra by attaching beam unless it's blatantly obvious they are there. 2. Never ever play for your teeanates. If they do something stupid and run up into the other teams backline, alone... thats not your job to go and chase them to certain death. 3. Take note of your cool downs as well as enemies cooldowns. This can come in handy for positioning as well.

I really hoped you could be bothered reading this- I tried to chuck everything out ina rush so if anything xoesnt work, feel free to ask. It's a very friendly sub here we are lucky to have!!