Can't believe no one's done a solution to this yet....
Edit: I didn't know Trueachievements changed the place for Guide's on achievements so I didn't notice people had actually done a solution for it already, sorry. I'll still have my guide up, though.
Anyway. This is pretty simple, you'll just have to do the last mission on über and the achievement is yours. Easier said than done, though. Some people have even said that they've been able to do all checkpoints before the last checkpoint on an easier difficulty and later changed it when arriving at the final part, resulting in them still getting the achievement. There kind of exists places where the boss can't hit you, on the far right or far left (the stairs), stand there and use the Raketen gun (pistol shooting rockets), dual assault rifles and for the Nazis, use the sniper rifle. Don't forget to aim for his mouth to do massive damage. My tip for surviving the whole mission on Über is to just speedrun it, I got to the last part in like 5 minutes doing that. Try to have as much health and armor you can on the final part, you will know that you're near the final part when you arrive at some catacomb looking place with three Nazis inside shooting through windows.
I'll link a video as well (not mine, credits go to CombatCow) Please comment if I've missed something. Thanks and enjoy your achievement!