So before I start obviously my advice will not be a solve all and make it seem like you are the best player ever you will improve the more and the longer you play the game.
So firstly settings are probably one of the most important things. The red settings are things that you will probably want to have like mine, this is because motion blur makes it very hard to see things accurately. Anti-Aliasing makes your game look like a cartoon in my opinion. Vertical Sync tries to make your game a lot smoother but comes at a cost and most likely will tank your FPS. And the blue settings are all preference except for maybe shadows but the other ones depends on your PC specs and what they can handle.
Firstly, location is key, know where you are and what you want to do. If you see a guy across a field you obviously won't run across it that is very obvious but it is just an example. Another example is you are behind a wall pinned down by 4 guys and cant go anywhere without getting killed if you are an assault class your smokes will get you out of the worst of situations since you either have 3 or 4 you could throw one at them and hopefully obscure their view and throw the other 2 on the ground and run to possibly a house get the high ground and again use your grenades to your advantage and rain hell on them and flank again to finish them off with your Thompson or Gewher. So what im saying is use your grenades to your advantage and if you are ever in a sticky situation throw a smoke while you are prone and try to find another way to get the them.
So as I said earlier try to find a different angle this will sound cheesy but, think outside the box. say you are trying to assault an outpost that is behind a hedgerow. There could be a wall that you can get on top of and towards the top of the hedges in game they are less dense so you can see through them. When you get close enough to the hedgerow if the op is right next to it you can get it through the wall and when you are on top of the wall you can see the peoples silhouettes and kill them that way instead of pushing them. And as always you can always use your trusty explosives and send them to the moon.
My advice on 1v1's or more than one person. So in this scenario you and another guy know exactly where each other are. You could peak the angle and hope you shoot him first or you can hold and wait for him to peak. Secondly surprise surprise your nice and trusty grenades smoke him out or blow him up but the smoke could bite you in the ass and he could use it in his favor. Thirdly think outside the box again find the high ground or find a different angle.
There is no advice for aiming in this because the only way to really improve that is to play the game what I hope you got out of this is that you need to use your grenades to your advantage and try to find a different angle try to out smart them with your location first then try to out gun them.