The Nazi UFO is the final level in the game. It's possible to use in-game cheat codes to unlock all levels and head straight there. More information on cheat codes here:
http://steamcommunity.com/sharedfiles/filedetails/?id=267608...Cheat codes are unavailable for the boss battle itself.
The boss battle is a 3 phase encounter. You can only damage the UFO during the intermissions where he will charge you from the right to the left side of the screen. The goal is to hit him enough to get rid of the symbols that will appear in front of the UFO when he charges you. I recommend getting all the way to the left of the screen so it takes more time for the UFO to charge you and kill you.