I created this guide to help people of all ranks rank up even higher. I specifically targeted this guide towards lower ranked people, but if you're at a high rank, you could still learn a thing or two from this.
It's annoying not being able to rank up. Getting stuck on losing streaks and losing matches because of poor synergy really gets on my nerves too. But there are a few tricks out there that you can use to really improve your game, and if applied correctly will get you at least one or two ranks up.
First thing's first: use motion controls. Seriously, use them. When I first started playing Splatoon, I didn't use them. I was used to the CoD controls, so of course I stuck with what I knew best. But soon I found myself getting stuck at C-C+ and couldn't rank up. I switched to motion controls and BOOM! I ranked up to B. Switching to them wasn't the easiest thing in the world. It took about 6 hours of play time for me to adjust completely, but it definitely payed off. I use almost exclusively shooter-type weapons (octoshot, forge splattershot pro, .52 gal, and .96 gal) and switching to motion controls drastically improved my aim and pan speed. Motion controls allow you to make very minute and precise movements with your reticle, which helps with aim in every weapon class, especially chargers. The only trade-off is that unless you have really steady hands, your aim won't be as steady. I've found that that is really easy to work around however, since you're constantly moving anyways. What's more is that you can combine using motion controls and the stick to increase your mobility and agility. Personally, I turn up the sensitivity very high so that I can use the stick for large and quick camera pans, while I use the motion controls to compensate for my lack of aim with the stick by making those small pans to aim at the enemy. You can combine them in other ways too. For instance, if an enemy is really close to you, but behind you, you'll need to turn your camera as fast as possible to get the splat before they splat you, so you can use the stick and motion controls at the same time to turn your camera super fast.
Another effective way to rank up is by doing research. You should figure out how many shots it takes for each weapon to splat an enemy, know their range, and know your surroundings. This is kind of broad though so let's zero in on each aspect first. First is the shots to kill. Each weapon has a certain number of shots it takes to get a splat. No matter how many damage-up perks you use, you can never decrease the shots to kill. It's a mechanic built in the game to make sure some weapons don't become over powered. If you know your weapon has less shots to kill than somebody else's, and you engage them, you are more likely to win (assuming you're of equal skill). Typically, the less shots it takes to splat, the shorter it takes to splat your opponent, but there are some exceptions. It's important to know how many shots it takes to splat somebody so you can plan how you will engage them. The easiest way of course is to go head-on, but if you use an aerospray against a .52 gal head on, they may very well kill you first. Knowing this, you could decide that it's easier to flank, thus winning the engagement. That's just one example of how knowing this information could benefit you and thusly your team. You also want to know their range. Basically, don't try to attack somebody who is out of your range. They'll see you and probably outrange you or retreat away from you; either way, you don't get the kill. Just be aware of others player's range, and plan your strategy accordingly. Finally, you want to know your surroundings. Go to recon mode of each map when you get the chance and find all the nooks and crannies in each map. Know where your weapon type is most effective, and use the map to your advantage. Get advantageous positions over your opponent before they can overwhelm you. Paint as much as possible. It's very important to get control of the areas you plan on using in the map, and control enemy areas as well, to make it harder for them to defend themselves.
Now let's talk about abilities. If you're not using the correct abilities for your weapon type and play style, then you're at a huge disadvantage to your enemy. Here's the deal: each class of weapons benefits differently from different perks. Chargers benefit more from damage up than shooters do. But shooters benefit more from speed boosts. This is just one example however. Once you have picked out the weapon you will use, you want to use gear that has abilities that compliment that weapon and your play style. There are five different classes of abilities: Physical abilities (damage up, defense up, etc.), agility abilities (run speed, swim speed, etc.), inking abilities (ink efficiency, ink recharge, etc.), special enhancers (special charge up, special saver, etc.), and tactical abilities (recon, cold-blooded, etc.). If it wasn't obvious already, just pick one or two of those ability classes and focus on rolling your abilities for them. Everybody has their own personal preference on abilities so just pick whichever ones help you achieve the most victories (sorry I can't guide how you customize your character).
Strategy. Strategy, strategy, strategy....This is by far the section i see people failing the most in. For me, it's the most annoying part of the game. In ranked battles, you have one job: go for the objective. If you are not going for the objective, you are either not helping your team or worse, being a detriment to your team. Of course there are other important things to do, like getting map control and fighting off your enemy, but you still have to go after the objective all the while. What do I mean by going after the objective? I mean that you should be trying to take the splat zone, ride the tower, or carry the rainmaker. It's okay if you're not doing these things however. Like if you're distracting enemies, taking their zone near the objective, and splatting them left and right, I would still consider that going after the objective. I can't tell you how many times a match is lost because only three teammates are going after the objective while the last one is lollygagging in the back trying to build up their special. It sounds like a good idea on paper, but once implemented causes the team to all get splatted while the enemy takes the objective. Your best bet is to just always go after the objective, and splat anyone who gets in your way. Almost ever match is won by the most aggressive team, so be aggressive! One interesting thing I have noticed is that there is a certain threshold of kills to win percentage depending on the rank you're at. What does that mean? Ill break it down. Splats do matter; that's incredibly obvious. If you're team never gets any splats, your win percentage is below 1. However, on the C- rank, if you get fifteen kills or more, your win percentage is drastically improved. The higher the rank, the more splats you need to increase your odds. At B-, you need 19 splats or so. At B+, it's 23. A+ is 30. It's very hard to dominate the ranks like that however, but if while you're striving for the objective you can achieve tat many splats, you will have about a 90% win rate, and rank up like crazy. How to get those splats? Read every paragraph before this one and you'll see.
Finally, the last thing you have to worry about is the most obvious thing: aim. We all know it's important, but not a lot of people have great aim. They compensate for mediocre aim with abilities, weapons, and timid play styles. To improve your aim, use motion controls, but more importantly, PRACTICE! All you need to do is spend a hour or so once a week in the weapon testing area practicing aiming in different ways at the dummies to get better. Every time you play online you get better at aiming as well.
Some final notes: if you get stuck in a rut, take a break. We all fall on losing streaks, it's natural. If you're losing a lot, change something. If you're not getting very many splats per round, change your weapon. Certain weapons do better on certain maps than others, so get used to changing it up. If you're on a winning streak, don't exit the lobby. Chances are, you're being placed on lower ranked teams. When this happens, just keep playing for as long as possible riding the streak, because the server will keep placing you with people on lower ranks as long as some of those people keep staying in the matches too. You'll typically win more in these streaks because since they're lower rank than you, you'll probably reach the splat threshold more often, leading your team to victory.
To sum it all up, don't forget the key points to ranking up: use motion controls, do your research, use abilities correctly, implement good strategy, and aim well. Some of these seem pretty obvious, but it's very easy to forget about them or just never use them. I hope I presented some useful information to people of all ranks, because this is the first time I've written a guide for Squidboards. Give me some feedback! If something is wrong or there's something I missed and should be added, let me know!