Enter The Gungeon

This achievement WILL require you to have made the Bullet that can kill the Past. (The first piece can be bought in the shop on Floor 2 for about 115 shells (please correct me if im wrong and ill edit!). Piece 2 requires you to find a special room on Floor 3 filled with a minecart puzzle (fairly simple, just get to the end of the room in the carts with a little good timing). Piece 4, on Floor 4, is in another special room that shows up with an invisible path leading to it. If you dont have an item allowig flight, its a slog, use your starter weapon to shoot everywhere, and use the casings on the ground as a sign of where the path goes in a new direction. The final piece in Floor 5 is the 'easiest' to get, you just have to kill the High Dragun! (but still, once you know his attacks, its not too bad). WHen hes dead, shoot his skull after the battle to reveal the last Bullet piece. Take all 4 pieces (which does NOT HAVE TO BE DONE IN A SINGLE RUN, SO DONT STRESS! It can be done over multiple runs!) to the Blacksmith/ Forgesmith , also on Floor 5, and she will construct the Bullet that can Kill the Past. NOW...once this is done,, go to her in all future runs, she will give it to you, and after killing the Dragun and finding the Gun that Kills the Past, you will go to your characters Past. Win the specific Boss fight with the Marine, Pilot, Convict, and Huntress each, and this cheve is all yours!