Watch this to see how to get Diancie using power save and this for the shiny lcked legends.And these are the other things you can do with power save:
You can clone Pokemon by backing up your save file, and then putting the Pokemon you want duped onto Pokebank. Then just copy the backed-up save file back to your cartridge, and repeat until you have the desired amount.
There are also other little cheats included with it, like injecting a Diancie Mystery Gift into your save file, which can be found in the interface somewhere... I've never used it, but it should be fairly user-friendly.
Make sure you actually have a 3DS Power Saves device, as you said you've had it for a long time, and it hasn't been out but for 3 months or so. If it's a DS game power save device, then it's worthless with X/Y.
Hope I helped!