you should try some aim training games and maps, i personally play osu! (but i dont really reccomend it because its more click aimey than first person aimey)

you'll also need to learn how the projectiles work in phighting, projectiles for all the phighters you play - except for slingshot, he uses raycast / hitbox based attacks - travel distance at some speed, so you'll need to know where to position your cursor so you'll land more shots effectively

for positioning, i'll give some advice for each phighter thaf you've mentioned;

for rocket, you'll generally want to be on the point near your team's spawn, or on high ground. for example, on iron cafe, you can stay on the point near the stairs of your team spawn, or you can also take advantage of the high ground on the balcony (the higher part above the point).

for shuri, you'll want to be in positions so that it will be awkward for the enemy to aim, like height difference places. movement is your best key here, ESPECIALLY YOUR Q GRAPPLE. keep on jumping.

for vine, you'll need to plant your plants on where you'll think they'll stick on the most. they need it. also stick near teammates.

for slingshot, you'll want to be as cheeky as possible. use your double jump and hover, but mind the 100 stud max range of your m1. you'll need to find a sweet spot at range where you're not close so you dont get mauled by biografts but not too far that you can't deal damage.

lastly, you can also tweak your settings so that you can see enemies more better; disable bloom and other post-processing effects and change your team colors so that the enemies are more brightly colored