That is a very useful and detailed write-up! The only thing I think should be added is that the right-side player should also call out any specials they see on the far-left 3rd floor if they aren't able to kill it in time. Give the left player a heads up. :)

It would be really great to have detailed descriptions like this for every position for a bridge hold, I would add that to the Strategies sub-forum.

Tips:
- Use throwables only when necessary, e.g., when you need to revive or defib someone. Save bile jars especially, these come in handy if someone gets incapacitated or dies on the ground floor somewhere far from the stairs.
- Adrenaline shots give a small health boost but faster movements. Helpful to revive players quickly if you need to.
- Shoot smoker tongues to save your teammates faster.
- If you jump when a teammate is about to get pounced, jockeyed, or charged, you can avoid getting stumbled.
- When someone goes down, run a circuit around the map to grab supplies (adrenaline, defibs, etc.), running will lead the tanks away from the downed player and allow you to come back to help them safely.


This is perhaps the most famous strategy in Left 4 Dead 2 and it is the most playable strategy for solo runs and unorganized public games. What's interesting about this strategy is that it can be done effectively with less than 4 players, even solo.

Weapons:
Any weapon is suitable for the circuit strategy, but some weapons are good in certain situations.

Autoshotgun - This weapon is excellent for running because it does not lose accuracy while moving like the other weapons do. The disadvantage is the extremely short range. You cannot save teammates that have separated when using the autoshotgun. Recommended for beginners.
M16 - This weapon is the most well-rounded weapon and good for all situations. The short reload time is very useful and the damage is good for short range and long range. Recommended for beginners.
Sniper Rifle - This weapon is great for sniping and saving teammates at long range, however, the accuracy is very bad while running. Also, the 3-second reload time is a disadvantage. It is still a great weapon for the circuit strategy but it takes getting used to.
SCAR - This weapon is powerful but the reload time is identical to the sniper rifle. It is more accurate while running.

Positions:
At the beginning of the round, all players start on the third floor in the wooded area referred to as the "saferoom". There isn't any exact positioning here, but if there are 4 experienced players who have played the saferoom strategy before, they would probably be positioned the same way. What's important is that at least 1 player is dedicated to watching the front and at least 1 player is dedicated to watching the back. If the front player positions themself half in front of the doorway and half behind the wall, chargers will have to come all the way up to the saferoom before they charge at you.

The players wait in the saferoom for the tanks to reach them. The tanks might come from the front or the back. Once the tank is there, the players go through the hole in the floor and begin the circuit.

After you drop down the hole and are on the second floor, run to the right across the second floor bridge and then go left towards the elevator. Replenish ammo on the second floor and keep moving forward until you reach another bridge. To the right, you run off the bridge and drop on the scaffolding and then onto the ground floor, and then head back up the stairs that takes you to the third floor and restart the circuit.

It is not necessary to kill the tanks that are chasing you because when executed properly, the tanks will never catch up with the survivors. However, if a tank has been lit and has ran a full lap around the map, they should be close to dead. It's better to kill tanks that are almost dead because if a tank dies in the saferoom and you begin running a lap, you're more likely to be cut off by tanks spawning ahead or above you.

Tricks to escape from tanks while running on Mall by AnGeR. Must-see!
http://www.youtube.com/watch?v=i1c020ARyZs
52 Minute Solo by AnGeR
http://youtu.be/oWWvco07Yik
127 Minute Running Duo by AnGeR & kris
http://youtu.be/KtyswaWZOPg

(Description stolen from guyguy's L4D2 Survival Guide! Viewable here. Also includes pictures and more videos!)

Tips:
- Go for ammo at 1:30, 2:30 (optional), 4:00, 6:00, 9:00, 12:00, 15:00, and whenever it's safe after that point.
- Use throwables only when necessary, e.g., when you need to revive or defib someone. Save bile jars especially, these come in handy if someone gets incapacitated or dies on the ground floor somewhere far from the stairs.
- Adrenaline shots give a small health boost but faster movements. Helpful to revive players quickly if you need to.
- Shoot smoker tongues to save your teammates faster.
- If you jump when a teammate is about to get pounced, jockeyed, or charged, you can avoid getting stumbled.
- When someone goes down, run a circuit around the map to grab supplies (adrenaline, defibs, etc.), running will lead the tanks away from the downed player and allow you to come back to help them safely.


As far as I know, this is the approximate strategy developed by TheKevSham.

Positions:
- 1 player left on the bridge by the ammo pile
- 1 player middle on the bridge
- 1 player, top of the stairs to the right
- 1 player on the stairs

The players on the third floor can use either M16s or sniper rifles. We like to have the middle & right side players using snipers so they can take out saferoom tanks mostly by themselves, and because the middle player usually doesn't have horde to deal with so they can deal the most damage to tanks while they're far.

The player on the left should use a good melee weapon to cut smoker tongues. Smokers can be really dangerous for the left-side player. Sometimes if you die by falling from the 2nd or 3rd floor, you can't get defibrillated. This player leads the tanks coming from the left across the bridge. When tanks are on the right, this player clears the left side so the other players can lead the tanks across the bridge safely without getting common locked. If there is a tank coming from the left and one coming from the right, to avoid getting sandwiched the left player will jump from the bridge onto the staircase to lead the tank away. Another thing the left-side player and middle player have to worry about are chargers coming from the left. There is hardly any time to react to chargers coming from the left if they spawn in the closet closeby, so this player will have to be really quick & very aware, and should also warn any players on the bridge as well if a charger is coming through from the left.

The player in the middle as well as the player on the left have the best views for most tanks. The middle player should warn the stairs player of what SI are coming up the stairs. Especially warn the stairs player of chargers & jockeys coming up. This player also helps the right-side player with tanks coming from the saferoom. If the left is clear, the left player can come help with saferoom tanks. There are some sneaky smokers the middle player has to watch out for, they can sometimes be on the 2nd floor behind the stairs, it's a bit dark so these smokers are hard to see. Another is on the ground floor at the bottom of the close-right pillar, they can be difficult to see right away because they smoke you through the stair railing.

The player on the right has to make sure to keep horde & SI on the right from dropping on the stairs player. If you get a tricky double tank and one is on the right, the right-side player can draw aggro of one and lead it back and forth along the hole, or lead it all the way to the saferoom and drop down if necessary. When tanks are coming from the left side, this player clears the right so the players on the bridge can lead it across safely without getting common locked.

The player on the staircase can use the Magnum because they might not always have a chance to grab ammo when they need it. If this player positions himself in the middle, about half covered by the railing in the middle, the jockeys will have to run all the way up the staircase, giving him more time to kill it rather than leaping on him immediately. If he's reloading when a jockey or charger comes up, he should just run all the way up the staircase to avoid getting pinned by specials. This player doesn't really do too much damage to the tanks unless they are in his view (climbing left/right pillar or on the stairs). The advantage of having a stairs player is they can bait tanks at the stairs, giving everyone more time to shoot. They also should be getting 40%+ of the common infected kills, making it easier for the players on the third floor to deal with tanks. Stack all the gas cans in the corner of the staircase for the stairs player to use after 13 minutes when there will be doubles about every minute.

104 Minutes by Licksore
http://www.youtube.com/watch?v=rYQPdiFuG70
Spectator footage featuring bridge hold strategy with a stairs player by guyguy
http://www.youtube.com/watch?v=mSrCbEguBIM


Positions:
- 1 player left
- 2 players middle
- 1 player at the top of the stairs

All players can use M16s or sniper rifles. It's helpful for at least one person to use the M16 to slow down tanks. I would suggest the player on the left and the player guarding the stairs to use M16s to better handle the common infected.

This strategy is very similar to the first one. I think it's a bit less organized though because there will be more commons in your way when you're leading tanks, and now there are 3 players on the bridge who have to sidestep chargers from the left. Another disadvantage here with not having a stair player is that the tanks will climb straight up the pillar, giving you less time to kill them.

The left player should have a melee weapon to cut smoker tongues. This player clears the left side when a tank from the right is being led across the bridge.

The 2 middle players should be doing the most damage to the tanks climbing the pillars and should also be helping with the commons coming upstairs when there aren't any tanks.

When a tank is being led across the bridge from the left to the right, the right-side player guarding the stairs should continue to clear the stairs if there is a lot coming up, but if there isn't they should back up with the others toward the saferoom and clear the path going that way.

116 Minutes by AnGeR
http://www.youtube.com/watch?v=_3jDpdRfGFY
86 Minutes, TheKevSham's gameplay, video by Azimuth
http://www.youtube.com/watch?v=AjM2dNnH5KQ


Credit to X-0ut for tweaking this strategy.

Positions:
- 1 player in front of the left closet (the closet closest to the bridge)
- 1 player left side of the bridge, close to the ammo pile
- 1 player middle-right on the bridge
- 1 player right, top of the stairs

Players can use M16s or sniper rifles, but having at least 1 player use the M16 is recommended because it significantly slows down tanks. I would suggest just 1 of the players on the bridge use the M16.

The core strategy has not changed so these descriptions will be short, I also took them from AnGeR's brief description on YouTube.

The left player at the closet is mainly there to block spawns out of that closet, as tanks and specials that spawn in there have always been a bit troublesome during bridge holds. This player tries to lead all the tanks that come from the back pillar down the bridge, while the left-player still leads car-pillar and far-left tanks down the bridge. They are able to warn teammates of a tank below on 2nd floor or if one is about to climb up the back pillar because they will feel a lot of shaking. Also having someone here keeps the left-player on the bridge safer as they are hindered by the Jimmy Gibbs Jr. poster which doesn't allow them to see what specials may be coming from the far-left if no one calls it. If a charger comes from the far-left, the closet player is able to aggro the charger and have it charge to the back corner, rather than charging on the bridge which can make things quite difficult for those positioned on the bridge, especially if they are dealing with a tank.

The player nearest to the ammo pile will also try to lead these tanks if the left player can't.

The middle-right player will probably be doing the most damage to tanks and getting the most SI kills since they don't really have many commons to deal with in that position. If they aren't busy with tanks/SI they can help clear the staircase. If your team communicates well, it'd be useful to have this player call out any specials they see coming up the stairs so the right-side player doesn't get surprised by chargers and jockeys.

Right-side position doesn't seem to have changed. This player clears the stairs and the right side of the third floor. They lead the stairs tanks and tanks that come from the right. They may have to kite tanks around the double hole to prevent tank pinches. Middle & right player work together to take down tanks from the saferoom.

164 Minutes by AnGeR
http://www.youtube.com/watch?v=Mwf4yXkeibE
218 Minutes by khan (WR) combination of stairs & left-closet
http://www.youtube.com/watch?v=bNmxWB1JAl8


Positions:
- 1 player back
- 1 player front-left
- 1 player front-right
- 1 player front-middle

All front players use the sniper rifle & a Magnum as their secondary. The back player uses the shotgun.

The front-left player leads the front tanks in and kites them around the hole. The other front players should back up sooner so they don't aggro the tanks. They all use the Magnum until they see a tank, that way they always have a full magazine to empty on it. If a tank is in the back, the front-left player continues to cover the front while the other two help shoot the back tank.

The back player sticks to using their shotgun the whole time, as there are not too many commons in the back most of the time, and they have to be ready for jockeys and chargers climbing up. They lead the back tanks in and kite them around the hole. If a tank is in the front, they make sure to keep the back clear. They can sometimes help with front tanks if the back is already clear or the front players say they need help.

For the first 12 minutes, grab ammo on the third floor bridge together. After that, you should do a full lap when the tanks come if you need ammo. We try to go when we have 1-2 magazines left.

If there are 2 tanks in the back, drop down the hole and do a lap. If there is a close tank in the front and one in the back, drop down the hole and do a lap to avoid getting caught between them. If there are 2 tanks in front (1 far and 1 close, or 2 far), they should be taken down just fine. If 2 tanks come from the front and are both close, it might be safer to just drop and do a lap. If there is spit at the front in the saferoom during a tank, it's also safer to drop and do a lap. What's really important is that the front-left & back players are skilled at kiting the tanks around the hole. The ledge they do this on is quite narrow, so it takes some practice.

We pile the gas cans in the closet closest to the saferoom and the back player and a front player grab one when they return from a lap if they don't have one there already. We try to only light tanks when we are going to drop and do a lap, but after playing this strategy a few times, I think it might be useful to light the front if there are 2 tanks and you don't think you can kill them that easily. I think this is ok to do because we seem to always wipe with lots of gas cans left. :P

This strategy is a nice change from holding the bridge, and lots of fun because it involves holding and running. Spitters tend to get stuck, so during the first 12 minutes when you go for ammo on the bridge you can kill them. They're on the ground floor, at the bottom of a pillar towards the back end of the map.

60 & 79 Minute Rounds by Lone Wolf McQuade
http://www.youtube.com/watch?v=2fWy_tB2qug
71 Minutes by Licksore
http://www.youtube.com/watch?v=auhrbEbK3XA