say your oath all you want for how i understand it the elements of harmony only work for the wielders on that boss once, like Celestia defeated nightmare moon and discord with the elements once, then she says in the discord episode if she tryed to use the elements on discord again it wont work for her thats why you twilight sparkle and your friends need to do it.
so what they are a 1 shot insta-kill for a boss only once, so if discord or nightmare come back there screwed as they only work on a boss once for that set, so unless you find some new ponys to wield them let the Chaos of my master roll XD
As far as I know, the elements wouldn't work for Celestia on discord because the first time she used them, it was with Luna, and when the elements were changed to accommodate the other ponies, they could no longer all be wielded by one person. So if Nightmare or discord came back, they COULD use the elements again to beat them. But that was just my interpretation.
by any chance did you ask the creator of the original picture if it was alright to upload this? Cause if not, you really should have as that's against DA's policy and you might wanna consider taking it down. Unless you did of which case you should also source the original creator of the picture