Stages are set up in a familiar fashion: equipment, ammunition stores, a trigger for the horde, and a safe house at the end. Unlike the first game, Left 4 Dead 2 has much larger stages. In some areas, this offers the survivors alternate routes when faced with certain dangers (e.g. witches).
Left 4 Dead 2 introduces a new challenge: unlimited hordes. During some triggers, you will have to run to another location to disable something that is making noise. Until that thing is turned off, the horde will continue to attack.
Throughout stages there may be places where you can fall and take damage or die. Falling off of buildings typically results in your character having to be rescued by another survivor.
Safe houses represent the end of a stage (except for the final stage in a campaign). When you enter a safe house there will be supplies for you to use in the next stage. Using any of the supplies now will carry over into the next stage (i.e. using two of the four med packs will result in only two being available in the next stage).
If all four survivors (or however remaining survivors there are) enter the safe house, closing the door will end the current stage.
There are regular zombies, and super zombies. In the fifth campaign, there are also armored zombies (regular zombies that cannot be shot from the front).
Hordes are large groups of zombies that are attracted to survivors. They are excited, and run as fast as they can to attack. Mini-hordes, or small groups, appear at random times or when the survivors reach certain points. A Boomer (Special Infected) can summon a horde of common infected by using its special attack & vomiting on survivors.
A large horde is triggered by special "Crescendo Events" during a stage. When triggered, mini-hordes will randomly appear in the area (there are usually only a few entry points and these can be learned to better strategize) until the event is over. Some events are timed, and others must be disabled by deactivating an object.
Special Infected represent the dangerous evolutions resulting from the disease. During the regular campaign (solo or co-op), special infected will try to attack the survivors at random times. On higher difficulties the special infected will coordinate their attacks.
There are an unlimited number of special infected in each stage. The longer you take to finish a stage, the more you will have to fight.
In L4D2, there are eight types of special infected:
An important thing to understand is that the entire experience is built around co-op play. That means you can't treat campaigns as a conventional shooter. You need your team and they need you.
- Play within the ballpark of your skill level. Don't go joining advanced-level games and grab the unsighted sniper rifle. Start in normal mode, melee bashing with automatic weapon backup. There's never enough melee, plus it's satisfying and the best entry weapon. Assault rifles and Uzis have incredible versatility as guns and you can never really go wrong grabbing one. If you frustrate your team they won't protect you, and the game is designed to stop a player of any skill level from being effective solo.
- Crouch. Even with an axe, but especially with any sort of firearm. Even with the desert rifle, shooting while moving from a standing position is laughably inaccurate. Crouching with an AK you can get headshots (not that you'll need them) in full auto, at long distance. Extra benefit: your team can cover you without pegging you in the back of the head.
- Move. You're losing a level badly as soon as you grind to a halt - there is no prize for camping yourself out of ammo before you die spectacularly. It just makes it a very painful process. The goal is the safe room. Try and cover your team as much as possible, but don't let a persistent camper drag the team down. Don't be the guy pinned in a side room half the level back looking for trinkets when you already have full kit, and don't worry about trying to save that guy.
- First Aid. 4 kits in every saferoom, by default it's pretty clear what should happen. If you have no kit, heal and DO NOT pick up a second kit unless there are more than are needed. Also NEVER heal above 65%, ESPECIALLY if doing so will deprive another teammate of a heal or backup kit they need. If you are +65% and have a kit in reserve, don't even take "yours" unless it's going to get left in the safe room. Doing these things is considered extreme disrespect amongst experienced players (unless you're being a team tank - which means doing the job, not just taking the most damage).
- Teammates. Be fast to cover them. Even in normal, a pin will kill a player in seconds. The AI director knows the easiest way to take Survivors out is one at a time, starting from the weakest link. That means if players are injured and slow, toss them some health so they don't fall back and get picked off. More importantly, it means leave yourself opportunities to break a pin on others. If a player is pinned, it's your #1 priority. Doesn't matter if there's a tank, spit, and fire between you. It's better to ragequit than give up on pinned survivors, unless covering a bad mover is too damaging to the team. Also pay special attention to who the director is focusing, and provide extra cover. He'll usually try and single out the best or worst performing player for an onslaught of specials.
- Fire. If you're a new player, don't use it or throw it at tanks only. Unless you are throwing at a tank, NEVER EVER throw it in front of the survivors, blocking their path. Even more importantly don't throw fire into a fight unless there is a tank involved - otherwise you'll damage the team, stop them, and split them up. Consistently bad/thoughtless/troll-like use of fire will get you TK'ed or kicked real quick.
- Use the environment. A single survivor can easily win a 3-on-1 special matchup with a nightstick if he's smart enough to run through a door and shut it. Similarly, consider your position relative to the horde and your weapon. i.e. if you have a baseball bat you want to be front and center (to cover teammates, to stop the horde stifling shooters.) if you have an auto rifle you want to be a bit back in the formation, trying to stay in positions where surrounding or even reaching you is hard. (need aim time, space to be effective, vulnerable to lots of surrounding horde) All this while try and pick weapons your team is missing (like melee, always melee), and refrain from "me-too'ing" specialist guns like sniper and auto shotgun because they become a liability when everyone's using one.
- Conserve ammo. Figure what it takes to drop a common, (in normal, 3 shots uzi, 2 shots m16, 1 shot AK, 1 shot shotgun.) then try and get your ammo consumption there. Also look to see what your teammates are doing - if they're dicing up a crowd of bile'd zombies, shoot the drifters that will run off at the end, take a foward position, or just leave your clip full for the inevitable post-grenade pin. Being effective means doing what your teammates aren't, not competing with them for what they're already doing just fine.
- Communicate and have fun. If you're noobish, just say so. Everyone has been there. In a lot of cases that will cause people to give you more leeway and protect you better.