You may not like to hear it, but if you are bouncing between B+ and A- rank, that means you are not on the A- level yet, at least in some map/mode combinations. It means the average player in the A- rank is doing something that you are not used to or prepared to counter.
Flanks will never really go away starting in A- rank, so it's important to learn when most people try to flank/shark and keep an eye out for telltale signs: only 3 team members approaching from spawn after a wipe or accounted for in a wide view of the map, or small blobs/trails of ink going through a relatively uncontested area on the map. Learn the common flank points and choke points on the map, learn to keep yourself safe from the flank points, and set yourself up to counter anyone that goes in hoping for a few easy kills. This is probably easiest on splat zones where the goal doesn't move around the map.
Make sure you're not always dying with teammates around you. If you're always the first to die, it means you are doing something that makes you stand out. Possibly you are in a spot that leaves you vulnerable, or you are not maintaining a good map awareness of enemy ink/locations (including snipers, watch out for the lasers *before* they approach you or it'll be too late). If you're always dying last and not realizing it until after you're splatted, it means you are not aware of your own team situation and pushing forward while you're alone. This is an almost guaranteed way to get yourself in trouble, as you have 4 members on the other team trying to hunt you down for that 4th kill. If this happens and you catch it, try to get somewhere relatively safe and wait for your teammates to come back (provided the objective doesn't need immediate attention).
If you're trading all the time, you are going after bad kills. Worse, if your enemy is running more QR than you, they actually have a slight advantage to trading with you! Don't engage anyone unless you are certain you can get the kill before they can react and kill you. Spend time working with all different weapons, either in ranked or turf, and get used to their range, fire rates, and damage outputs (shots to kill). This is vital information that can help you decide whether or not to engage a specific enemy player with your own weapon later on. Also learn the range of each weapon and when you need to be backing up or pushing forward as you attack, to keep the enemy at a disadvantage. Long range shooters are a little less viable close up, usually due to a slower fire rate or lower damage output than short range shooters, so a short range shooter will often have the advantage vs, say, a dual squelcher or .96 at point blank. Similarly, weapons like the jet squelcher can outrange most shooters and blasters, so use that range to your advantage by putting yourself in a spot where you can shoot the enemy but they cant shoot back without getting closer.
When you see a team's "coordinated" pushes in solo queue, it's most often just one or two people reading the situation on the playing field and reacting in the best interest of the team's needs at that moment. For example, if I'm playing a match and notice that everyone keeps dying around me when they enter a specific spot on the map, and I have a special that can help break through, I will take the time to charge it and then use it during my team's next attempt at breaking back in. If the RM carriers keep taking out my team, I will try to shark in a relatively safe spot where I know the RM will come by (that doesn't give them too many extra points, if any) and get the kill that way, to ensure my team's constant failed attempts don't affect my efforts. In TC, I always run an invincibility special so I can force a few extra points on the tower on my own, and in SZ I will run splatlings or weapons with inkstrikes/bomb rushes - anything that can help me regain the zone as a single person.
These are just a small number of tips I've gained from working my way up to S rank personally. I too was stuck in the A/A+ range for a long time, and it got frustrating for a while. You can improve, but your hail mary tactics may be getting your team into more trouble than you think. Always analyze and strike when the odds are in your favor or the clock is running against you, and you can maximize your chances of a successful carry/push/whatever. If you have a way to record your own gameplay (either with a cell phone/video camera on a tripod or a direct game capture), record your matches and watch it back. Try to understand how the match unfolds as you watch it over, and make mental notes on how you can avoid your deaths or unsuccessful pushes in the future. Another benefit to recording these is that you can also post your videos and ask for advice (something I've done more than once to the tune of some great advice on things I didn't really notice myself).