If you come across an unfortunate problem where when you try to load your save file, you see the Aveline/Animus loading screen but then the game goes back to the title screen, it means the game has glitched. This is a problem that Ubisoft were aware of and they have issued a patch to fix it, which you can download by connecting your PSVita to the Internet.
Unfortunately, while the patch fixes the problem, it does not allow you to continue your glitched save and the only option is to play again from the start.
The best way to get this trophy, which is for killing 7 guards on rooftops in the Lady Persona, it is best not to use ranged weapons like the parasol or poison darts.
Instead, go to the Governor's house and aggro as many guards in the area as you can. Go round the back of the guard post and head up the staircase, following it round until you are up on the roof. Kill all the guards that have followed you, and, if it is not enough, head back down and lure some more up to their doom.
Assassin's Creed III Liberation is designed to be played as Aveline, but if you own the PlayStation 3 version and hook them up, you unlock a mission where you can play as Connor on the PSVita version. Choose the Extras option in the menu and then connect it up to your PS3.