You firstly have to build the Trample Steam which you need:
The flag pole
The brown air thing
These can either be found in the starting area or in the room next to the starting area (You pay 750 points to open the door). Before you can build it though you need to turn on the power which can be accessed a variety of ways but the best way is to go down the lift at the start, jump to the next building and open the door in front of you to get to the power switch. Once you've done that use the elevators to get to the roof, then jump back to the building you started on. Now, all you need to do is pick each part up then go down to floors on either of the two elevators here to reach the workbench (This is much easier with 4 people).
Once you've built it just set it up with the up button on the analog stick and wait for it to pull back, zombies will be attested to it so make sure they don't break it. Once it has been pulled all the way back simply watch the zombies in front of it get flung into the air. Repeat 10 times in one game.
i done this one on solo same as demonhunter said but if you just 750 points on round one open the door and find the 4 parts for the table which is 2 floors down so you can just fall down the gaps at the end but go floor by floor otherwise you will die and just wait for the quick revive to come down so you can get back up to get the other parts, it takes time but its really easy once you know what your doing and also if you are still on round one and haven't killed anymore zombies once you have made the flinger you can put it down facing the dragon and fling yourself across and get the facing the dragon head on achievement. if you need anymore help add me on xbox
Although in co-op if everyone is going to agree to do the build ASAP, it is only one door still (follow comment #4 on the Facing the Dragon achievement) it is just a lot more distance to travel but with more people you can carry more pieces of the build at once to the table.
All achievement guides are going to upload soon. Got 6 out of 10 upload already will upload the turned ones later or tomorrow and the easter egg i am working on. If your stuck for any of these acheivements feel free to check my videos out on my channel.