Round 1 - 5,

Each person takes a window or 2, and do the shoot and stab routine. You can let them come in the windows or not, your choice, but while you might get a max ammo, you could get a bomb which is going to take away some possible points. Note: shooting in the extremities (legs, arms) will allow for more shots to be put in the zombies from the pistol. Around level 4 or 5 someone needs to open the door to the elevator. Buy the MPL on the wall and someone go ahead and clear the debris. Sit at the elevator for these rounds and kill with the MPL down the hallway. I like to shoot a few times with the MPL then stab, unless there is a pile coming. Hit the elevator when you feel unsafe or run out of ammo for the MPL. If you have 3 people, you should be able to rack up 2-3k easy per person before hitting the elevator if you are playing as a team of 3, but this is with rebuying MPL ammo off the wall. (I've went downstairs as early as 4 or as late as 6).

Round 6-10,

If you haven't hit the elevator by round 6, go ahead and hit it to go downstairs. Now in the command room, go to the right and buy the MP5K off the wall. If you don't want to get rid of your pistol to upgrade it later on, trade the MPL, but I generally go ahead and trade it in when with 2 or 3 other people. Stay upstairs for a round or two to rack up points. Head down when each team member has a few thousands (generally we have 4-5k). Don't clear the debris in the control room yet, just the one to go downstairs and the one at the elevator. When in the basement (lab), find the mystery box and open that room. Hit the mystery box until your out of points hoping to get a ray gun, a lmg (HK or RPK), winter's howl, monkeys). Any decent SMG (AUG, Commando, Galil) will be sufficient for these rounds though.

Head back to the elevator and stay in the elevator to rack up points. One person watching the window, the others out at the elevator doors shooting. Remember, if you ever get in trouble, just hit the elevator switch, then switch it back down on level 2 when they start overwhelming everyone. Near the end of each round have the window watcher throw a grenade through the window to make a crawler at the window, so everyone can go spend points on the box. When the box moves, if it isn't in the lab (level 3), just take the elevator upstairs to the box, or if someone has a ray gun, then go ahead and hit the power and teleport upstairs.

At about round 8, if you haven't turned on the power, turn it on. If someone has a ray gun, then you can stay on level 3, but if not, you'll need to go up to level 2, before the start of each round to make sure the scientist isn't coming. If someone has a ray gun you can stay on level 3 and camp in the elevator. If you see him, he's coming for you (if he hasn't taken a weapon yet from you). Pull out your crapiest weapon and unload on him. Then chase him back to the elevator and unload while he takes your friends gun. The person with the ray gun will need to unload (while chasing after him other weapon has been taken). Generally, the person with the ray gun (upgraded or not) will be able to take him out.

Now if you don't have a ray gun, go to the second floor on level 2 (control room) and stand on the catwalk with your team spread out. If you see him coming up the stairs. Get your worst weapon at and start running while your teammates unload on him while he's chasing you. Don't worry about shooting him, just run. You'll be able to run from him for a while at full sprint if he's chasing you (you are going in a circle). Rinse and repeat until he's killed. This is the only way I've found where you can kill him without a ray gun/winters howl/upgraded weapon.

Rounds 10-15

If you haven't got juggernog yet, you need to. Get it asap. Also, upgrade when possible too (hitting all the defcons on level 2). Now by round 15, someone should have a winters howl or winters fury if they've upgraded. This will be a must at 20+. In these rounds use the same elevator strategy, while getting certain perks at your discretion. Perks should be bought in order of Juggernog, Speed Cola, Quick Revive, then Double Tap.

Rounds 15-20

By now each person should have pretty good weapons (Winter's Fury, Ray Gun, HK, RPK). In between rounds (crawler left out), someone needs to go set all the defcons, except the one closest to the elevator. This is the escape plan. Also, the debris on the lower level of the control room need to be cleared. Repeat the same elevator strategy for these rounds, but if you get in trouble, use the elevator to go to level 2. One person will need to go set the last defcon, and each team member go through the teleporter in the control room to the pack a punch room. If you have monkeys, throw one right when you get out of the elevator to give everyone time to get to the Pack a punch (and to set the defcon to turn it on). After everyone is in the pack a punch, the person who has the winters fury needs to get about 5 foot behind the doors that open, and everyone else behind him. After the doors swing open, fire off about 4-5 winters fury shots. It will freeze the entire pack of zombies and will kill them. Now run around second level until all the zombies have spawned and kill them off by turning around and shooting every once and a while. You can also go to the elevator if needed to get back to level 3 and camp up in there to kill them, hitting the elevator when necessary.

Rounds 20-25+

Repeat the strategy above. Buy a mp5k off the wall and upgrade it if ammo is a problem (which it will be later on). and change your route to the catwalk portion (if you need to buy ammo) when all the zombies have came through the windows. Be sure to reset all the defcons except that one when you can. Use the winters fury with efficiency, like all the speciality guns on the maps (thundergun, wunderwaffe, etc.) ammo in these guns are precious, but powerful.

Tips: Make sure each person has a mic. This can't be stressed enough. Each time you hit the teleporter call it out. If you are teleporting, say "I'm teleporting." Teleporting at the same time will down both of you and at best cost you perks, at worst it will cost you the game.

Fire from the hip with LMGs or SMGs when at the windows. This will allow you to rebuild and shoot at the same time.

Use the monkeys wisely, you might need it to revive a downed teammate or to escape from a sticky situation, especially on five.