On RailsG 50

On most of the levels in the game, this achievement just simply isn't possible to accomplish, so it will have to been done on an easier stage early on. You can only be hit by an enemy twice, the third time you die. On most of the levels there will be multiple times where you will come across barricades and exploding enemies in the center lane, and since the achievement calls for staying in the same lane, dying is unavoidable. There is more than 1 stage it can be unlocked on however. I was able to get through stages 1-3, 1-5, and 1-7 without changing lanes. It was easy on these 3 stages so just play one of them.