After noticing I'm a little out of practice with AoS, I challenged Tank and Thegrandmaster for a quick 2v1 in pinpoint to keep up my skill with wall duels and I decided to write up what I learned.
Cutting to the chase: Efficient wall dueling.
Wall duels are probably the most important aspect of AoS shoot-outs. Nine times out of ten, if you're fighting an experienced player, you'll be facing a wall duelist at his element and this is not a good position to be in, unless you know how to do it. Wall duels mostly shape up as two opposing two-block high walls you can crouch below for cover, quickly popping up to take shots at the opponent. The players will be strafing along the wall, releasing crouch at different locations every time to make them much harder to hit.
The key is to actively try to match the pace of your opponent. You should try to pick up on his rhythm at the first two pops, or you'll be looking at the spawn timer before you realize it. Be fast, and don't try to get a shot off every time you pop up. If you find yourself trying to aim for the other guy, you are dead - you need to work on instinct, a twitch aim, if you want to win. Be unpredictable. If your opponent can guess the location you will pop up at, you die. When facing an experienced wall duelist, you will see a lot of tricks and shenanigans to throw you off the ball - and when you lose the pace, guess what, you're dead again. You'll be seeing crouch-strafes from the sides of the covering wall, and possible jump-shots if there's a higher portion of the wall at about 3 blocks height.
So, you've got the drop on your opponent, you've learned the time it takes from him to take the shot after he pops up. Start taking quick shots at him, and crouch before he shoots. Against Tank, tGM or any other skilled player, the whole ordeal of popping up, shooting and crouching again has to happen in half a second or less. If you're taking the time to aim instead of twitching for the shot, you're most likely walking out of the spawn zone again. Always pay attention to how close to the wall you are. If you're too close, your gun will be poking through, telling your opponent your exact location. If you're too far, the top of your head is vulnerable to a jump-shot from your opponent.
So if it's so easy to die with wall duels, why choose it over any other form of cover?
A two block high wall probably offers you the best versatility available in a shoot-out. Since standing shots with the rifle are just as accurate as crouching shots, you can utilize the standing position for gaining the maximum strafing speed. Keeping yourself scoped in is not really recommended to keep up the pace, but if it works for you, go for it. Shooting no-scope does not penalize your accuracy either, though, so you'll need to experiment to find your style. Remember to stay a few blocks back from the wall and keep moving, and you'll be fine.
Crouch-strafing out of cover exposes you a lot more than a quick release of the crouch-button. You can accomplish a full routine in a much, much smaller time frame than what it takes to strafe out of cover and back. There's also the brief moment of complete vulnerability when you crouch-strafe out of cover, as your opponent can see a small part of your player model before you can even see him. He can also get a shot into your head before he's visible to you.
For the love of all that is not Nicky Minaj, never use a three-block high wall. If you cant see over it while standing normally, it's bad. People can easily run up to it, and hide behind it on a public game. Building one may seem a good idea from a defensive perspective, but in the end, it always just ends up helping the opposing team to run up, as people cannot shoot the rushers straight off the spawn. Cover always works to both directions, and being on the open side of a high wall is a much more favorable position, than to be the guy who runs through the small opening in the wall. Your situational awareness of what's behind the wall is severely limited, and you'll most likely just get shot by a guy you never saw.
Another little death-trap people often use in pubs, are low bunkers, with 1-block windows at crouching level, and a roof on top. In case you need to get out of there fast, you die. Being limited to strafing at crouch/scoped speed is a certain death against anyone who knows which end of the rifle to point at your way. A great way to offer your opponent a few kills more, though.
And lastly, the walls with the little fortifications on top. If you strafe behind them, it's easy to lead you in to the next little slot on top of the wall, and shoot you before you even see out of it. If you stay still in a slot, you'll get shot from the fog-line with the help of your tracers with zero effort. Applying wall-duel tactics will probably end badly, as it's possible for you to occasionally pop up behind your own cover, making you lose your rhythm. Guess what happens when you lose the rhythm?
I mainly wrote this off to solidify the bits of experience I got out of the practice game we had. Perhaps someone else will find it useful, hence why I'm putting this out in the public domain.