I can throw in some tips, as I play both autoaim and freeaim matches.
For free aim, some of your don'ts become very decent strategies:
Flashlights: From a distance, flashlights lack glare or blinding capabilities, but still present an obvious white dot. For any snipers, this is an easy point-and-click kill, but I find snipers to be rather lazy and relying ton the susceptibility of their prey to get them kills. Since you're going to be setting them up, it doesn't matter. Now, this is what I do to get most of my kills:
There's a wonderful series of townhouses that are near the LSC shop in central LS that can be accessed from a ladder. I sit up here a lot. This presents lots of advantages for me: First off, I'm on high ground, but I can still make a quick escape without killing myself from fall damage. Second off, I have a great view of the street. The streets are elevated, so I'm looking down on them from my position. Lastly, it's across the street from another great spot, a car park, so I can hide in there if I need to. Here's where the flashlight comes in handy: As I said before, many players rely on trying to predict their target's movements. If I stand straight up on my roof and turn my light on, my enemy now assumes I'm an idiot who has just given him or her a clear target. Usually they start blasting away with an AR, and that gives me time to roll back before they get any hits. Then, they reload their gun while they wait for me to reappear...
Reloading is death. Never reload your guns unless you're CERTAIN there's no enemies around, or you have less than three bullets left. If you're practiced with headshots, you should be able to nag one with anywhere from thirty to ten or less bullets. I usually expend 10-20 bullets with the AP pistol or Advanced Rifle when I shoot for the head, and this is usually because of their high fire rates. Reloading during a fight leaves you vulnerable, extremely vulnerable. You have forty bullets left in that extended mag; that's at least three more kills if you're smart about them.
Back to the rooftop. Now that the guy below is reloading his gun, I roll right back and blast at them. This works for me 9 out of 10 times. Flash, roll, roll again, unload. Usually they'll try to roll themselves, but that can screw them over even more:
When you're reloading, you can do it faster or slower. Reloading and then falling, jumping, or vaulting can reload your gun automatically. This isn't MUCH faster, but it can be handy, especially if the area you move to gives you decent cover. This can also work with rolling; start a reload, then roll. When you get back up, your gun should be reloaded. For rolling, though, THIS IS NOT ALWAYS THE CASE. Sometimes, you'll still have to reload your gun when you get up. For example:
Say I'm shooting someone with a standard SMG. I use up all 30 of my bullets trying for a headshot and he slips into cover. My guy starts autoreloading at the same time that I try to roll. By now, the enemy has started shooting at me. When I'm back on my feet, I haven't reloaded my gun and have now spent about 5 seconds being shot. I'm dead. Only reload when you're sure it's vital.
Let me also throw in a few details about some 'underdog' weapons. There's one in each category, besides the Heavy weapons.
PISTOLS: Pistol .50
This is for owners of SE/CE versions only, unfortunately. a .50 cal Desert Eagle, much more powerful than the .357 found in IV. This pistol, when used accordingly, can be one of the most lethal weapons in the game. As soon as I could get onto Online, I got some guns. I got the bullpup shotty, hammer, and of course, this pistol. This gun is free in the store, as are all paints and accessories. This is what kept me alive through levels 1-50, and then some. I never even touched the AP pistol until I finally decided to own all the guns. This gun, for players with health up to 40/100 can kill in 3 shots max. Combine this with the rather fast fire rate, this gun is perfect for catching off guard players and getting headshots. This gun is extremely powerful and accurate, and should definitely be kept around.
This is the gun I see being used the least in Online. It seems no one touches this, as they've already unlocked/found a 'better' gun by the time they notice it. The SMG bridges the gap between SMG and AR, and does it greatly. This is one of the most accurate automatic weapons in the game, especially with a scope. It also can hold up to 60 bullets, and can be confused quite easily for the Micro SMG from a distance. Speaking of the Micro SMG, the SMG pools its ammo with it, so you can get ammo for this gun for about half the price of a pack of ammo for an AR.
ASSAULT RIFLES: Carbine Rifle
Despite the oxymoron in its name, the Carbine Rifle is a very, very well stacked gun. Many people (especially in autoaim games) only rely on this gun until they get the more potent Advanced Rifle, but seem to overlook the staggering accuracy of this rifle. Basically, the first five or six shots you shoot from this will go exactly where you want them. The accuracy drops decently after that, but you shouldn't need more than that to do some decent damage allowing for a burst. This gun is excellent for players who can't stand the scope of sniper rifles, but still want to snag long range headshots.
SNIPERS: Bolt Action Sniper Rifle
Both the snipers in the game are about equal, but for players who play a lot of deathmatches, this will likely be your preferred rifle for one giant reason: you can silence it. This hides you from the map and truly makes you a stealthy assassin. keep in mind that if you don't manage a headshot, you won't do much damage and will likely lose your target if you aren't keen on follow up shots.
SHOTGUNS: Sawed Off Shotgun
Many, MANY players (myself included) love the Auto Shotgun, but the potency of the Sawn Off is still to be reckoned with. This gun is a one hit kill in close range regardless of health. The range is actually quite decent, and can allow for some quick hits if you're caught off guard in cover. It can also be shot from bikes.
THROWN: Tear Gas
This can be especially lethal for players who never bothered with it. They won't know what to do when their health starts trickling away, and will sometimes run even further into the gas until they realize. By then, it'll be easy to drop them with a few shots anywhere.
Hope this helped.