Every other post on this sub is something about how scavs are "OP as hell" and are one tapping people from across the map with 12 gauge. Not to say the latter doesn't happen ever, but it sure isn't happening as often as y'all make it out to be. So, after replying the same shit to 20+ posts like this, I figured I'd make a post.
Scavs have a FOV cone in front of them that allows them to spot players, as well as an audio listener that gives scavs a general area as to where loud noises are (such as sprinting or gunshots). The FOV cone extends a fair bit to their sides too, so be careful when trying to sneak up on one. However, there is almost always a delay in the time from them spotting you, to them firing their first shot. That window is an opportunity to take them out before they become a threat, and if you miss that window, you're now in an engagement. Don't stand out in the open, don't crouch thinking they'll miss if you crouch, run for the nearest cover and break their sight lines. From here on out, the Scav is aggro'd onto you and will shoot at you on sight.
Now, this will vary depending on if it's a regular scav, or a raider. I'll cover normal scavs first, and then raiders. Please keep in mind that these are simply tactics that usually work out for me, and don't guarantee anything.
For scavs, hide behind cover for a couple of seconds before peaking from a new angle. I'm not sure why, but by breaking LOS for a certain amount of time, the scavs seem to switch into a different mode where they're back to searching for a player, but actively hostile. By making them enter this mode, more often than not they'll have the firing delay when you re-peak them. Again, this delay is a window to take them out with a well placed headshot. Never peak for more than a couple seconds. This opens up many little 'windows' where you can fire without taking any hits yourself. Alternatively, you can use this tactic to find avenues of escape if you're in no condition to fight, since typically scavs won't bumrush you. If they do, peak them and open fire. Sprinting delay still applies to AI, and they won't be able to fire while running.
Raiders are a different animal. With their heightened aggression, they're much more likely to completely obliterate you if you're in the open when they spot you. If you think you're near a raider, get to cover ASAP. Then go through the same motions as the normal scavs, but this time only quickly poke your head out. Raiders will usually spray at the location they last saw you, and once that spray is over a window of attack opens up. Raiders are also far more likely to sprint from cover to cover, or even right up to you. Use this to your advantage and get as much damage on them as you can while the window is open. The most important thing to remember is that you shouldn't rush killing a raider. Take it slow, and make every movement and action you take have purpose. Additionally, raiders will usually run away from grenades. So if you've got a grenade or two, throw them where you believe the raider is to either A. Blow them up if they decide moving isn't an option, or B. Use the window opened by them running away to put some shots on them. When fighting large groups of raiders, I play it exactly how I would play a player engagement. Take them on in a series of 1v1s and dont open yourself up to more than one at a time, if possible.
Adding on to raiders: Don't headpeek first. Jiggle around so your arm leaves cover first, and let them shoot that. Once the burst at your arm is over, your window is here.
Sniper Scavs are also sometimes tough to deal with, however they're a fair bit more clueless than regular scavs at times. Their FOV cones are a considerable bit smaller compared to normal scavs, but the distance they can see you is greatly increased. Taking up a side angle on a sniper scav is, in my experience, the most effective way to take them down. If you can't get a side angle, then quickly peak from one angle to draw their aggro there, and then peak from a different angle to give yourself time to land a headshot. If your first two shots don't bring it down, then repeat the process. Never peak from the same angle twice, otherwise you're asking to be shot.
Really, a fair bit of the things I've discussed here can be applied to any engagement. Play it slow, and keep the ball in your court while keeping the enemy guessing as to what your next move is. The moment you begin playing predictably is the moment you're putting yourself at a disadvantage. And finally, the most important piece of advice I can give you is that you won't win every encounter you have. Dying is a part of the game, and sometimes a situation is too far out of your control for you to swing it in your favor. The only two responses you can have to these kinds of engagements is either backing off and picking a fight elsewhere, or dying, taking a deep breath, and committing yourself to earning that kit back in the following runs.
If anyone has some advice they'd like to be added here, I'm all ears. Or if you think I'm stupid, I wanna hear that too. I'm not a perfect player, this is just what has worked for me.
Edit: I've also noticed scavs can kinda "sense" when you're aiming at them, so be cautious of that.