The single most effective combo is good headphones and microphones for everyone. With a good set of headphones you'll hear the specials coming in, where your friends are in trouble, the direction of the horde, etc. There's nothing more satisfying than hearing the spit from afar and jumping up onto a bench or table before it even lands or meleeing your teammate while the smoker's tongue is just starting to reach them.
This also includes always knowing where the exit is located. Seriously, when you get on an airplane they point out the emergency exits. You should be doing the same when you play. Where are the objects you can jump onto to avoid a spit? Where is the door/window from which you can bail when a surprise tank spawns? Where are the high points from which a hunter can pounce on you?
Call for Help
If you find yourself overwhelmed, lost, or incapacitated, call for help from your teammates. Don't wait for them to figure it out on their own. As you get better at playing together, you'll often be able to sense when someone is about to get into trouble and can go help proactively. Until such time, speak up!
Also, if you're deviating from the path or see something of note, give out a shout. Someone may have missed the horde or tank music or didn't see the ammo pile.
There is a not-insignificant delay between you saying something and your friends hearing it, somewhere around two seconds. Think about all the trouble you can get into in two seconds in this game. This is how people get killed by getting smoked backwards after their three teammates have dropped down from a ledge. This usually happens because the first person yells "Okay, drop!" right as he's dropping through the hole. Everyone else hears the order two seconds later, but the guy watching the rear doesn't see his teammates all drop.
Learn the Maps
Know where all the weapon drops are. Learn where you can find melee weapons, health and throwables. On campaign mode you have time to shop for goodies. As you get to expert and realism, you may find it starts costing too much. Often the person in front can do a little shopping as long as you keep an eye on them.
Often with my friends the leader will step off the path to pick something up while the others pass by and then rejoin the group in the rear. The key is that everyone knows this is happening and that you don't all do it at the same time.
Pick your route before you start the map and follow it. Are you going through the park or to the right or left? Decide early and only call an audible if it's warranted. If you change the plan, make sure everyone acknowledges the change or makes it obvious that they know. There's nothing worse than having a straggler get picked off and lose a kit or die because they didn't know the rest went left instead of right to fight the tank in the open.
Mix Your Weapons
Unless you're all ace shots with the hunting rifle, it helps to mix up the weapons. If you have someone good with the shotgun, make sure to take one even in tier one. They can clear large hordes as long as they're fully loaded. Having at least one person with a hunting rifle or military sniper is great for picking off smokers and dealing damage to a tank throwing rocks from the roof (less common in campaign).
Conserve Your Ammo
I have the horrible habit of double-tapping the auto-shotty on hordes. I do it with the AK and M-16 which is perfect, but it's a waste with the shotgun. I make up for it by killing what I'm shooting at and not firing unnecessarily. There's nothing worse than being stuck with a single pistol when a horde shows up.
Conserve Your Stamina
Shoving the horde back to give yourself time to reload, escape, or get a better shot is very helpful. Since you fatigue after a few shoves, make sure you don't overdo it. And when you do, swing in an arc to get more bang for your buck.
Kill the Specials
All but the charger can be knocked off your friends with a shove, but that's often not the best option. While taking the time to kill the hunter mauling your teammate might cause them to lose another 10 or 20 HP, batting it away just gives it another chance to pounce and do more damage--perhaps at a worse time. Take advantage of the fact that they are a stationary target and unload on them.
If you're close enough to shove, switch instead to your melee weapon and take them out. I lost count of the number of times I've put four or five point-blank shotgun blasts into a hunter without a kill. Sometimes it just doesn't register. Then again, I've had the same thing happen a few times with a melee weapon. :(
The worst time to throw a pipe bomb is right when you hear the music for a horde spawn; you'll only kill two or three. Wait until the horde is close to you (twenty feet or so) and then throw the pipe towards the horde to allow as many as possible to surround it before it goes off.
If you're going to throw bile, be smart!
- Out of the line of travel. Never directly in front (duh) or behind you (swimming upstream).
- Into a fire if possible. Mmmm, barbecue!
- Don't be there. Seriously, chuck that vomit and run like there's no tomorrow. It calls a horde, but that's fine if you're no longer around to see it.
Fire is your friend. If the horde is spawning from a room or over a fence, drop fire between you and them and keep booking. Make sure you save at least one molly for the tank, though. :)
Most important: Call your throws. There's nothing more annoying than a pair dueling pipe bombs or having someone throw a bile right after your pipe. I call these out every time I decide to throw, and whenever possible I give two seconds before I throw to account for the radio delay.
- "Pipe out"
- "Puke out"
- "Fire out"
When it comes to lighting the tank on fire, call out ahead of time who's going to throw the first molly so you don't waste fire. This matters more once you start playing versus where the tank will actively dodge the molly and try to trick you into throwing it from far away.
L4D is about moving and stopping, moving and stopping. Whenever it's clear, you should be moving your ass. But when a horde comes or in an event, it's time to crouch to the ground with your backs to a wall/corner. If you can funnel the horde through a choke point, put someone with a melee weapon in front, crouched and swinging away, while the others stand and shoot over their head. Knowing that the melee weapon can kill most of the zombies, only fire on specials or if you start to get overrun.
Gather Your Assets
Most events will have gas cans and/or propane tanks lying about. These are perfect because you can set them up before starting the event and save your throwables for emergencies. Don't waste a gas can by throwing fire on it or put them so close together that lighting one lights them all.
Be careful if you store gas cans where you're cornering up! A careless shot while turning to pick one up to throw can ruin your day. This is more important in survival, but I've seen it happen many a time in campaign and versus. Your safe little corner can quickly become your grave.