Lothar is the third german you fight when the Germans invade. You are meant to die at this point, usually during the first german; So it is extreamly dificult.

These notes have been given to me by user id/Vic=HKC=
And i'm kinda just posting them here so it's easy to go to the overlay and see them for myself :S
I'll edit this if/when i acctually get this achievment personally...

• If their energy is above 40, I tended to block the potential attacks, especially if my health is under or equal to 40. Heavy attacks make you loose 40 HP but takes also a lot. Besides, blocking an attack is only making you loose 5 HP. Between 40 and 5, I choose.
  
• Backing down to rest is also a good idea, if your opponent's energy is under 40. He will not attack. However, above, he could charge.
  
• Also, sometimes, they'll go forward but end up without energy. Attack!
  
• If my health is above 40, I tended not to block.
  
• If you loose, reload. It can happen, even with the tips above. But it was what worked the most. I even managed to get a good health before fighting Lothar.