You can make this a lot easier by preparing your class before you start a game in "The Final Reich".
Choose the
Camouflage and
Serrated Edge to become invisible to zombies and deal more melee damage. In addition, you can buy the Faustblitz perk in the bunker to increase melee damage even further.
---------------------------
The first step is to turn on the power to get in the bunker. Here's the Pack-A-Punch Video that shows all necessary steps:
---------------------------
After the power is activated you can access the salt mines where you can find the bomber zombies. They will also show up regularly in later rounds. Activate your special with Lb+Rb to become invisible and take out the bomber zombie with a few hits.