This achievement is titled "Duh Dun DUNNN!" and it is for seeing the true ending of the story. This is a bit of a pain, given the fact you have to go in to story mode and complete each of the Canon endings with the available characters, then the 2 that are unlocked from doing that and finally complete the new option for the "True End".
There are a variety of paths the stories can go, for each of the stories refer to this link in order to know how you must advance for your Canon Endings...
http://blazblue.wikia.com/wiki/BlazBlue:_Calamity_Trigger/St...For the most part it's options in text dialogues, the occasional dimensional drive/Astral Heat finisher (Bottom of the special moves list, uses the gauge) or even winning or losing a fight.