I've been playing this mode on and off for the last week with a friend of mine and I've come to some conclusions as to how to survive longer. Here's some tips I would very much like to share:
1] Easiest one first but it's surprising how many people don't know this. When rebuilding the barricades you don't have to be facing them. You only have to be standing next to them and hold the rebuild button. A lot of people I have seen simply stare at the window while it's being rebuilt only to lose their head by a wandering band of zombies who snuck in behind him. This way you can cover your rear while making vital repairs. You can also shoot at any zombies at the window while repairing, so you can get a few extra rounds into them.
2] Reload at every opportunity. Even if you've fired one or two rounds; reload. I've lost count of the number of times I've squared up against a horde and had the last bullet in the magazine get that vital headshot that sees you survive the round. The is especially important with the lumbering life-savers that are the MGs.
3] Leave the door closed. Opening that door is asking for trouble. Best thing to do, I have found, is to clear both staircases of obstacles and make the "Random Chest" room your alamo. You can have one person easily handle the "Random Chest" room on their own for the first 10-12 rounds while the rest have a party upstairs. This gives the zombies more access points but it also forces them to sometimes use the longest route to get to you which filters out their numbers and makes them more managable.
4] The flamethrower is useless. As tempting as it is to use it, just leave it alone. All it does is make the horde of screaming zombies a horde of flaming screaming zombies. You'll be digesting in their stomachs before the flames take them down. Same applies for the molotovs. Not worth the hassle.
5] Stick together. When you get past wave 10 or so, it is wise to stick in a group. There is nothing worse then having to fight your way across the whole map to revive a teammate.
6] Revive as a team. When someone loses an ankle the person who has to reattach it is very exposed and will usually need to be wearing Huggies. It is best that one person revives while the other two protect them. And as soon as that's done, run like merry hell to the alamo and regroup.
7] Instakill is your friend. It might not make much of a difference early on but in the later stages it is as if the heavens themselves have parted and given you the most holy of gifts. If you have a raygun; switch to solid rounds. Not only does this save precious raygun ammunition but it is very satisfying to shoot a mass of zombies with a machine gun. I tend to have the FG42 as my backup as it puts a big grin on my face. Only downside is that when the raygun dries up it takes a magazine and a half to put down a single zombie at point blank at wave 20 and above.
8] Communication is vital. When I play with my friend we have our PCs next to the PS3s so we use xFire to talk. We've found that this gives us an edge over others when we do competitive games. It allows us to coordinate without the others being privvy to what we're saying. Downside to the console versions of the game is that you cannot type messages if you're not using VoIP. Also, many people have their mics in and all you can hear is them chatting mindlessly to a mate in the same room or coughing their lungs out. So you mute them and when they do have something to say you can't hear them. Not very helpful. But, if you're playing with three other people you know VoIP is fantastic and will help warn the others when situations become dire.
Nazi Zombies was put into the game with the intention of it allowing players to experience a fully cooperative game where you will depend on one another. Bit hard when no-one can talk to each other. I have found that sending a quick message in the early rounds advising not to open the door can help out a lot but what is not needed is having one person with a mic who thinks they're in charge because they're the only one who can talk. When someone issues me orders over VoIP like that, I mute them. Again, not helpful. Best thing to do in this instance is simple situation updates.
To reach a high ranking spot you need to be able to have four players that can talk to one another without annoying each other.
9] Grenades. Bit of a grey area this. Chances are you'll end up blowing yourself up which, last I checked, is very counter-productive. The only time I use grenades is when I'm closing up the concrete wall. Just as I'm about to repair the last chunk, I lob a grenade in to take out any undead heathens that might be waddling along. After throwing it I then close up the last piece. That way the zombies in the tunnel are blown to teeny tiny bits and I'm protected from the blast. Throwing grenades is usually to be used in desparation i.e. running as fast as your virutal legs when the starting room is overrun by zombies that move so fast it shames 28 Days Later.
This is just a small bunch of tips I can think of which, I believe, will get any group of players past wave 25. I, myself, have only reached the 23rd wave and we failed when one of our number fell above the alamo and one of us ran upstairs into, what can only be described as, a wall of death. This left my friend and I screaming downstairs trying to survive until the next round began. Canute has an easier job, believe me. We reached that wave with only two of us being able to communicate. I strongly believe that if all four of us could talk we would have broken the current standing record (on the PS3 version) which is currently the 28th Wave.
Would like to shout out to the members of said game;
infilTRAITOR88 (myself, natch)
ChaosBubbles (my friend)
Broken_Splinter (who was only level 1 and made a bloody good account for himself)
Now to hope they release new levels for this mode. Games can last for an hour or more and I always finish sweating. It's very intense. This is what gaming should be. Plus it reminds me of Dog Soldiers. Which is neat.
I hope this helps some folk out there!