The Forgotten City

The fastest way to earn this achievement is to start a game with a soldier class (receiving a gun with 10 bullets) and once you enter the city:

1. Shoot Galerius (the first NPC you ever encounter after entering the city). Make sure you don't miss, as you need to shoot him 10 times in total.

2. Then you will receive new quest "RUN / FOLLOW MAGISTRATE". There will be a marker on your HUD showing approaching target - DO NOT SHOOT HIM. Allow him to sacrifice himself in order to open up a Time Portal.

3.Enter the portal. Repeat steps 1-3 until achievement popped.

To answer additional questions that I myself looked for while doing this achievement:

a) Do these stack between characters?

THESE DO NOT STACK BETWEEN CHARACTERS. All 10 loops must be on one character. I tested this myself; having 7 loops on my first character and 4 on my second, I had to load a save from my first character and loop 3 more times.

b) How else can I do this?

You do not have to shoot / kill Galerius for the time portal to open. You can do any sin / crime, as long as Sentius lives long enough to open the portal for you and you enter the portal. For that you can steal anything, harm someone or admit to purposefully killing someone that died in an accident. Fastest/nearest thing to steal would probably anyone's chest in Slums part of the map. If you want to get rich on your way for this achievement, I recommend stealing Aurelia's key on the first floor of Tavern and then robbing her chest with over 2000 denari.

c) I completed the game! Can I still get this?

Yes, just load any of your saves and SIN, then go to the portal - rinse and repeat.