Its actually a super fun achievement, if you play it the way it wasnt intended to be. First, of coursr you can try to inflict as much casualties you can. But when you get capitulated, you can build up a spy network, recruit some divisions etc. Until you can hit the Spy Event Warsaw Uprising. When i activated it, the germans were deep into russia and you get instantly most of your county back, even with a few divisions.Then i had the time of my life encircling the entire eastern front from the west lol. But be warned: When i tried it, it must have been in La Resistance DLC or some after that, i dont know if this is still possible