Are you focusing on a specific line of League matches or are you doing all of them?
When it comes to main focus of teaming, if you are very defensive, and need offensive options, it would be wise to be at least inking range of your Brush ally, as you are co-op with that person most of the time, inking up walls and staying on high places and constantly keep moving so you are within range of your ally to protect them and so on, that should be your main goal when both of you are alive so you can work together to get to the target in the League matches.
Splat Zones:
Best bet here is to nail down any opponent that is on your side of the map and staying out of range from the zones, if you have the zones on your side and the opponent are down you go in and try to turf and outrange them, know which opponent have longer range than you, keep clear, if the opponent have the zones, you stay out of range, and try to harass turf the zones from a distance, they have to waste their time, turfing it back up, or try to splat you, in that manner, your brush can try to go from behind and attack them there, your special would be the Tenta Missiles, keep aiming those missiles on the enemy in the splat zones, and nowhere else, if you have special and your team is down, go back to spawn, fire the missiles when your allies respawn and then you keep going with what you have been doing so far.
Tower Control:
Since you are using a long range weapon for this, you should have your brush friend on the tower and you support it from a distance, most of the time the opponents are gonna do anything to blow of all competition for the tower, and you make sure they don't get on the tower at all or shoot them off yourself, if you are riding the tower, make sure you turf and aim on areas where enemies can come from and keeping them off from there at least, if they arrive at the tower, bail off and get better position to fight back, preferably on high places or farther back in your own ink.
Rainmaker:
It's very iffy here on what to do, since you are defending and attacking at the same time, if you are just belining for the rainmaker and not turfing, your squelcher may not be that useful at all since this mode is all about speed and mobility, but if you have the rainmaker or at least have taken it away from the opponents, destroy any other nearby, if you are free, turf turf turf turf turf, you don't want them to come back to quickly, so turf them away from the rainmaker, as your weapon may not be that good to protect the rainmaker from close range, especially when you may end up just shooting your teammates backs instead, if the opponent has it, stay in an area where you have an easy way of firing on it to prevent it from going further, charging after it is bad since your mobility is not good while you turf, so stay safe, and stay high when you are on the defense.
Clam Blitz:
I'm not very good myself on this as it requires almost an entire team to just score once, but you should play defensively when you the opponent have a big clam, and offensively when your ally has one, remember that the entire opposing team can see where the big clams are in your team too, and if someone is holding them. Juggle the ball between areas and such, to make sure you don't hold it all the time and reveal your location too much, if your allies have them, stay in inking range, and try taking off the opponents going for them, also if your ally with the clam goes down, there is still time to pick it up later, just keep splatting the opponents, when they are no one left or the time is going out for the clam, grab it and throw it somewhere else, preferably backwards to your base so other allies can pick them up.
Strategies here can very a lot, as it keeps changing depending what the opponents are doing and where they are.
That's the main gist i have since i mained the Squelcher myself at one point.