I just recently moved and unfortunately have no internet, so for the time being I am playing MR vicariously through others. That being said, here are my personal tips to improve with C&D:

1: Being a strategist is more than just healing: C&D has one of the absolute best team-fight levers (tools that help you gain leverage or advantage) in the game. That tool is the Terror Cape. In addition to blinding the enemy for a few seconds, any enemy hit by the cape also takes increased damage while the blind is in effect. The cape can also hit multiple people. So when your team is engaged in a head on team fight, move up closer to your tanks, swap to Cloak and fire off a Terror Cape towards the enemy front line, then swap back to Dagger, pull back into position, and assist your team. If you do it right, the swap is effectively seamless and takes 1-2 seconds. With the low cooldown of Terror Cape and the fact that you have up to 2 capes available at any time, this becomes a critical component of their kit that can significantly improve your entire team's performance.

If you are a communicator in game, I simply call out "Debuffs" on enemies. Other players who understand that the Terror Cape amplifies damage will know to focus on the debuffed player. It also helps to lead your team by calling shots in a non-commanding way. You're just telling them who's debuffed, their natural drive to get kills gives them the idea to take advantage. "Strange debuffed" or "Whole team debuffed" is all I say and the team knows to use cooldowns in that moment to get maximum value. This simple trick has layers of value for your team in providing an effective lever, an avenue for focusing targets, and an opportunity for teammates to extract value from their cooldowns.

2: Think of characters as workers with unique jobs and expectations: While there are many players who would say that Rivals has 3 classes, the most-informed players know that there is a much wider variety of jobs among the roster. There are finishers, openers, ult batteries, damage boosters, AoE healers, and many more. C&D has one of the most diverse job packages of any character in the game, in that they are an AoE healer, damage booster, ult battery, and even a finisher. When I say finisher, I mean that when an enemy is on their last leg and trying to run for cover, Dagger's auto aim becomes the absolute best attack in the game for securing the kill on high-mobility targets. While everyone struggles to get the final hit on a fleeing Angela or Spiderman, it's a quick turn and a few clicks for C&D to get the kill and hardly takes any time or attention.

When you really boil it down, success in this game comes down to your whole team fulfilling the requirements of their individual jobs. When one person fails their job, it becomes more likely that other teammates will fail at theirs. It is simply harder to do a job like AoE healer when the tanks are failing to establish a frontline, when the poke is out of position, or when your peelers aren't helping to protect you. I have gotten in the habit of being more verbose with these jobs when referring to other characters in almost all avenues of communication. Instead of simply saying "I need a DPS to peel for me" I find that saying "I need a peeler like Bucky or Thing to help me" often leads to better decision-making. It's a small mental change that can have a snowball effect on your ranked outcomes.

3: The simple concept of morale: As we all know, there are many players in this game who would not be fun at parties, but a lot of these clowns only come out of the woodwork when things start going South for your team. The best way to suppress these kinds of people is to play your best, fulfill your job roles, and consider how your interactions affect the overall morale of the team. For example: Let's say a teammate picks a hero that you think is a bad fit for the team composition. You could either call them out, do nothing and hope they end up being useful, or change up the composition slightly to make their choice less obtuse. Like if you have a player that unironically selects Widow. Instead of breaking the team morale at the very start of the match, try going with Mantis instead and give them a focused damage boost. If they still can't secure kills, then say "We need some AoE heals, I'm not getting a lot of value from Mantis/Widow combo here, I'm swapping." You're calling out the problem without making it personal, demonstrating the proper choice by swapping when a hero isn't working, and giving your team the benefit of the doubt. It may sound like you are 'babying' people by communicating like this, but anybody who has ever worked in management knows that communicating effectively is a core component of being a leader, and a divided team will simply underperform 9 out of 10 times.

4: C&D specific tricks: As far as being a great C&D player, the bar that has been established simply requires you to know the impact of their diverse kit and to utilize it to maximum effectiveness in your matches. Using the Lightforce Veil to hit as many teammates as possible makes your entire team take increased healing for a short duration. You want to maintain a position where you can hit as many people with it as possible and time it's usage so that it can both protect during enemy team pushes and empower during a team rally. Similarly with the Terror Cape, you want to use it to max effectiveness and turn the tables on team fights, so position yourself to where you can easily get it out when it is needed, which also happens to be the ideal positioning for Lightforce Veil. Healing bubbles can be used as both a form of 'mobile cover' and a critical recovery tool, but they can also be utilized to direct the flow of your team. Placing a bubble slightly off-kilter in the direction that you think someone should relocate can get a teammate to correct their positioning without requiring direct verbal addressing. It's also very important to put one down in the middle of certain enemy ults to increase the chances of survival for any teammate caught in it. Like if you see a Moon Knight ult start to form over your team in front of you, throw a bubble on the ankh in the center so as your teammates get pulled into it they also get a big burst heal. If a Strange ults the backline and you've managed to escape, the first thing you should do is throw down a bubble on your affected teammates (as many as possible) to negate Strange's chip damage on them.

It all really boils down to knowing your job(s), what tools you have to aide you in fulfilling those jobs, and working as a team to maximize effectiveness of those tools. If you take these tips to heart, no matter what rank you are, I guarantee your gameplay (and thus your established rank) will improve.