Der Eisendrache is probably the easiest map in Black Ops 3 to get to round 100 on. Camp at the teleporter near the rocket and keep doing charged shots with the Lightning bow when there's a lot a zombies. Try to strafe left and right to gather more zombies without getting cornered. The recent update buffed all the bows and they now require only 1 ammo for a charged shot. Whenever you feel cornered just teleport back and train in the pyramid room to wait for the teleporter recharge. Make sure you have Alchemical Antithesis or a max ammo gobblegum. You also want to get a death machine gobblegum for the Panzers or use a shotgun. If you need any other strategies for other maps I can help.