Okay, so if you're looking at this solution it's either because no one else's was doing it for you, or you're looking for some more tips. Well here it goes. Keep in mind that this is how I got it, but I will include notes as to how others managed to pop this bad boy.
Firstly, you need at least two people (duh) to play this. Now, I got it with four players but I also noticed that when trying it with just two, the crossbow seemed to show a lot sooner and more often while the box also tended to spawn closer. This may simply be the luck of the draw but the choice is yours. The only advantage of more than two player is that you have that it takes longer for the zombies to get you out of the game.
I also recommend doing this on the Kino der Toten map, mainly because that's what my guide uses, but also because there's more space, more circuits (see "Training"), and Monkey Bombs seem to appear more often here than in other maps. It's also a lot easier to open up this map than "Five."
You need to be pretty skilled to do this achievement so I recommend you work on a few pro tricks to increase your life-span and point accumulation. The best way to practice is to play alone as it makes the stakes higher. Frequent XBL on occasion so that you can also get used to working in a group.
1. Courage: Okay this isn't actually a skill but the zombies can be pretty intimidating. Learn to tame your fear and focus on surviving.
2. Running: You'll be doing a lot of it. Those who camp, die. I don't care how good you are at locking down sections, or how far your friends got while in that one corner, if you camp, you will get trapped, overrun, and killed. It is extremely helpful to know how to find breaks in a zombie hoard and sprint through it, minimizing your chances of being cornered.
3. Training: It's not enough just to run, you need to train. Training is a tactic by which a player, or the group runs around the map in a circuit (any circular path, or open room will do) and thus gathers the spawning zombies into a cluster running behind the player(s). What's really useful about this tactic is that after a few circuits, all the zombies will be in one spot... on your six. This makes gunning them down a lot easier. Also, in Kino, if you get a group to stand in a corner while one guy trains, they can pick off zombies from the herd as you pass them, and considering zombies are fixated on the player nearest them, only a few, if any, will stray towards the camping group.
4. Knifing: Black Ops sucks when it comes to knifing as the default has you do a stupid stab animation which opens you up for attack. Instead you want to knife by pushing in the right thumbstick and simultaneously pulling back on the left thumbstick. Doing this right results in a quick and defensive slash animation which is much more practical.
Carpenter (The Hammer): Never pick this up if you can help it, as it repairs all barricades, making it impossible to get those last few desperate points for... whatever it is you need.
H-Bomb (The Bomb): Avoid this like the plague as it will only net you 400 points. Zombies can accrue more than that. Only pick it up if 1)it is at the beginning of a later round and zombies can still spawn in, 2) It is between rounds and can't do anything but benefit, and 3) you need to revive a squadmate.
Insta-Kill (Skull): I'd avoid picking this up unless you are getting overrun or don't have Juggernog in the later rounds. If this is the case, use the opportunity to cram as many knife kills in as possible.
Max Ammo (Box and Bullets): Call out for people to reload and pause a few seconds before picking this up. Also, if someone is downed, avoid grabbing this as downed team members will NOT benefit.
Double Points (X2 symbol): If you can grab it, go for it, but if you are worried you'll go down, don't bother. You're better off alive.
Juggernog (Red Machine): Get this as soon as the theater is accessed, the power is on, and you have enough. It will boost your health to 200% and is literally a life-saver.
Quick Revive (Blue Machine): This is in the lobby and will speed up your revive time, but it will not revive you. It's invaluable if your team is constantly getting downed, and can be well worth the 1500 it costs but only grab it if you don't have the crossbow and there is someone who doesn't have Jugg or skills.
Speed Cola (Green Machine): This one is found in the third room just before the dressing room. It will boost your running speed, the time it takes for the mystery box to cycle weapons, and even your reload speed. If you have heavy weapons you absolutely need this. It's a rather steep 3000 so plan accordingly.
Mule Kick (Green Machine): This is found just upstairs from the Speed Cola. It gives you the ability to carry a third weapon. It's 4000 to buy and frankly, not really worth it if you're going for the achievement.
Weapons: Don't buy any weapons off the walls except the MP40. The box's weapons often trump anything on the map and is cheaper too. Yes, it's a gamble, but if you can't risk it, grab the decent MP40. Trust me.
Right, now that you have all the skills, time for the actual achievement acquisition.
Round 1: Your team should pick a window, (or a couple depending on players) and pop six rounds into the lower torso/legs of oncoming zombies. Doing this will net you points without killing the undead hordes. (Pro Tip: even pistol round penetrate multiple zombies. Conserve ammunition by hitting rows of zombies rather than singling them out.) After you've pegged every zombie, let them break down the barricades and saunter in. It's the first round, chill out. Once they are inside, knife them to finish them off. If you get downed here, you suck. This way zombies may drop pickups (max ammo) and you'll be able to farm points by rebuilding barricades.
Round 2: Do the same as Round 1 only, drop a whole magazine into each zombie before knifing.
Round 3: At the end of Round 2, have the second highest scoring player (2P) open the doors at the top of the balcony (750pts). If the box is in that small room you opened, hit it, but not before the highest scoring player (1P) opens the next set of doors (1000pts) leading into an open room with two sets of stairs. If the box was not in either of these rooms simply grab an MP40 (1000pts) off the wall and have two teams: one on the ground floor and one on the second/balcony area of this room. There are three windows here and the stairs that zombies will come from. Whoever has the stairs will also face the bulk majority of zombies, so make it a good player. You'll all want to stay here until after the dogs round.
Dog Round (Randomized rounds 5-7): Camp down in the corner next to the Speed Cola machine and let them come to you.
After the dogs, have 1P and 2P open up the next two doors (1250pts each) to the theater and turn on the power. (Pro Tip: If someone tells you not to turn on the power, they are idiots begging to die, as this WILL leave you trapped and surrounded.) Once the power is on, go to the far back corner of the theater and you'll find a Juggernog machine (2500pts). Grab this first, and then hit the box (950pts per use). If the box is nowhere to be seen, the board on the walls will show you where it is. If the box is in or around the alleyway and you are still on rounds 8-10, don't worry you're still in good shape. To access the alley and other areas around it, open the ground floor doors in the lobby, NOT the one on the stage. (Pro Tip: DO NOT OPEN THE DOOR ON THE STAGE!) Keep hitting the box through the rounds until you get the crossbow. Make sure to train zombies and create crawlers at the end of rounds so you can take your time boarding windows, hitting the box, buying perks, and deliberating strategy. These moments are crucial. Once you have the crossbow, have that player save up for a Pack-a-Punch (5000pts). Link the teleporter using the machine on the stage and the pad in the lobby. Once the crossbow wielder has 5000 go up the teleporter, PAP that badboy and then wait to get back to the lobby. It is best to to this at the end of a round. Once this is done, ready the teleporter again (you'll have to wait for it to cool down first) and run over your strategy.
The way my group did it was to start a fresh round sitting in the teleporter, activating it once the zombies were spawned and coming after us. While in the projector room, fire a few shots to weaken some zombies while waiting to teleport to the lobby. Once in the lobby, a player ran forward to the rightmost panel of the desk (at the entrance of the theater) and told the crossbowman when to fire. My group did this on round 14. The first bolt missed me but the second hit and brought the achievement with the explosion. Another successful attempt later confirmed that past round 10, two bolts are needed to kill zombies, not just one. As such I recommend the crossbowman invest in Speed Cola (3000pts) if they can afford it, as it will help their reload be faster. Note that shooting downed allies does not count for this achievement. I noted this as a possibility and other confirmed. If things go wrong, run and keep trying.
Another strategy I've heard is to use Monkey Bombs to get zombies in a distracted group so the "Lamb" can get in the center of them. However, this requires more box purchases, and possibly more waves of enemies. Still, they're helpful for reviving teammates.
It has also been suggested that since zombies are non-responsive while the teleporter is in use, that the bolts could be fired from the balcony at the "Lamb" who would be able to situate himself amidst a gathering, despondent zombie horde. You're free to experiment, but I found this method to work just fine. So good luck.
Please comment or critique if you did not approve of this guide.