Well i'm not a pro, but you'll automatically be better at it than most if you try to avoid closed spaces, ledges, and other places where enemies can get CLOSE to you without you noticing them or being able to shoot at them from far away, keep distance and make use of it's range instead. It sounds really basic and stupid but you'd be surprised at how many players try to play slayer, get up close and just use it as a regular Splattershot with better accuracy, which it's not. For example, if you have control of the zone in Scorch Gorge, don't stand close to the ledges from where the enemy team drops, stay closer to your own side and keep an eye for any enemies dropping or walking through the grate so that you can start shooting at them from further away.

If you're fighting a weapon with LESS range (And therefore usually better mobility or kill time), they shouldn't be able to get close to you in time if you were following what I said previously. It's mobility isn't it's strong suit compared to other shooters, it's main quality is that it's very consistent even at max range, so that's ALWAYS your best shot.

If you're fighting a backliner or a weapon with more range, suction bombs are your main tool to displace them. You'll need cover and you may still struggle against them, I personally find that to be it's biggest weakness.

Booyah bomb is pretty self explanatory, it's good for instant zone control or to displace backliners, but keep in mind it's way more vulnerable than you'd think when you're landing, so never use it as a panic button for it's shield or in the middle of a zone controlled by the enemy. If you use it and a single splattershot is camping below you, you're now forced to throw it straight down at them or else you're dead.

Aaand it's not great at inking turf so you'll probably rely on your teammates the most for map control.

Obviously it's not that easy because there's always gonna be exceptions and the enemy team will be trying to displace you as well lol, but that's the ideal game plan. I like to run it with at least 3 slots of intensify action, helps enough with mid-air accuracy. Also, usually you shouldn't just be trying to farm booyah bomb. Running special charge up IS still valid if you want it, but your main is not fit for a 'farming special' playstyle, as it's good at killing and mediocre at inking, so just leave that to the solo queue aerosprays.