I've been looking at guides all day, but nothing helps.After completing Yaschas Massif -01X AF- I unlocked Augusta Tower 300 AF. I completed the Tower, got the right password, and got to the point where it said I had to go to the past to find a key to this Elevator. So I leave the Tower. Nothing happened. I go online to look up what to do, but all the guides said that After all that I had done, I should've unlocked Augusta Tower 200 AF, but it didn't unlock for me. I kept searching to no avail. Can anyone help me?

You need to find an artifact to open Augusta 200AF. Here is a video showing where all of the wild artifacts are in the game 

Apparently I was supposed to go to a time that I hadn't visted yet, from a Gate that I didn't see, and from there, I go to Sunleth, and then to Acadamia, and then finally, Augusta.

Oh God, I had the same issue when I was playing the game, and I had stayed stuck there on that same segment, lol.  I don't exactly remember how I got to the other Augusta Tower since I beat XIII-2 at least six or seven months back...so yeah, I know that you have to open a timeline in one of the already existent ones, and the one you unlock is Augusta Tower.  Yeah, I'm probably not making sense right now, but I hope you understood what I said.

Oh God, I had the same issue when I was playing the game, and I had stayed stuck there on that same segment, lol.  I don't exactly remember how I got to the other Augusta Tower since I beat XIII-2 at least six or seven months back...so yeah, I know that you have to open a timeline in one of the already existent ones, and the one you unlock is Augusta Tower.  Yeah, I'm probably not making sense right now, but I hope you understood what I said.

After getting my plat on XIII-2 i have no desire to replay it anymore