I thought I'd make a post about the way that I play this map, as I have the first 7 or 8 rounds on this map down to a science pretty much. I can get the Storm Bow done by round 5/6 depending on the dog round, and I find it really helpful when doing the easter egg to start off with this strategy.
Don't kill any zombies yet, take the tram fuse, turn on the landing pad and then prone in front of QR for that extra 100 points, but don't buy it
Wait for the tram to cool down and call it, best case scenario you'll get a double points, if it's an insta kill or nuke, don't bother with it
Maximise your points by killing 5 zombies (8 shots into the leg then knife)
Open the door that takes you to the lower courtyard, and if you had a double points you should have enough for the next door. If not, prone in front of Double Tap and shoot 8 rounds into its leg but don't kill it
Lead the zombie up to the first Dragon head and knife it underneath it to activate it. Also pick up the zombie shield part in this area
At this point you may have no ammo, so just maximise as much as you can by knifing and making sure you let every zombie go into the Dragon's mouth (you should have 1 left by the time the Dragon is done)
If you got that double points at the beginning, you'll have enough to open into the living quarters and then the door after that into power. If not, continue to maximise points as much as you can until you have enough.
From this point, I'll continue the strategy assuming you got a double points at the beginning, as the only difference is that you'll be around 6 zombie kills behind
Of course, turn on the power and then continue down to the Undercroft. On your way, get the zombie shield part in the upper courtyard. Don't do the anti gravity pads yet, it just makes your life harder for the Dragon head I believe
Get your 8 kills underneath the Dragon in the Undercroft, preferably with your knife. You should have 5 zombies left, and you should be able to put the anti gravity on easily enough. Get the zombie shield part and then go up to the lab area.
Build the zombie shield upstairs, and then kill the remaining 5 zombies under the Dragon there
Kill the remaining 3 zombies required for the last Dragon head, and do not kill any more. Run downstairs and pick up the bow, then run to the gravity well to get back to the upper courtyard (saves running by all the zombies still alive in the lab area)
Shoot the weathervane and collect your arrow. Then shoot your 3 bonfires and do the antigravity step.
Come up to the urn above Double Tap and get your 5 kills here first. For an easier escape, clear the debris beside the Wundersphere. Try not to kill any more zombies than necessary here
Go to the clock tower and get your 5 kills here. You can shoot the bonfire for this and the Double Tap urn now if you want to, however you can leave it if it's too awkward for you with all the zombies
Open the door to the teleporter and teleport. Fill the urn and shoot the bonfire. This will most likely end the round. Don't forget to teleport PaP and activate the landing pad. At this point you can optionally stay at the Rocket Pad to do the Ragnarok part, however to continue with this guide you would have to train the zombies and avoid killing any, which can be difficult in that room without Juggernog.
Whether you have done the Ragnarok part or not, either teleport back up or use the wundersphere and shoot the final two bonfires. I appreciate that this can be difficult with so many zombies, but if you are able to master lining these up, you can run in and do these first time. I recommend watching MrRoflWaffles' video on it, I'll link it at the end of the post
Go up and put the arrow back into the weathervain, and then pick it up again. Then claim your max ammo and fill the soul box to get your bow. This can often take the whole round and a little more, so be patient with it
So this is my strategy for the first 5ish round on Der Eisendrache. I may end up extending this guide with my low round strategy for the boss fight, which can be done very easily by round 16/17.
Here's the MrRoflWaffles video I referred to for the bonfire shots:
I'm aware that I could've explained this myself, however I feel that a video is much better for this specifically than a text tutorial :)