I've been toying around with this deck. It can certainly handle itself admirably in a lot of match-ups.
I am unsure why the battle compressor though? Is it just to use a discard/seeker engine for particular supporters? I suppose a little thinning of the deck occasionally too - but to me that seems like two wasted slots that could be better spent on more fairy drops or max elixir maybe. Forcing the BC engine in, when there's no other part of the deck that it has any kind of synergy with is just that - forced, IMO. More often than not, a BC in my hand was a dead card.
I didn't have 4 Diancie when I started playing this, so I went with a 3/3 split between it and Xerneas. Once I went up to your 4/2 split, I found I missed the extra Xerneas, and didn't need the 4th Diancie? More chance to open with a Xerneas the other way, and less reliance later in the game on the DCEs if you are all set for single fairy energies. This would tie in with abusing Max Potion since you can move the fairy energies off for a full heal via Aromatisse, where you can't do that with the DCE. The deck in general could benefit from a more single energy approach instead of DCEs. If you went this route you could even sneak in a Wonder Energy or two - which you can move around with Aromatisse to block all sorts of Status Effects and even a few "place damage counters" attacks. That will shut some decks right down. of course you run the risk of it being discarded, but you have the same issue with DCE already.
The only other suggestion I have is an oft overlooked card that works really well with Aromatisse decks, in Exp. Share. Attach it to your Aromatisse/Octillery/Jirachi when you can, and that's one extra energy on the board to be moved around at all times.
Suggested:
-1 Diancie-EX
+1 Xerneas (Geomancy)
-2 Battle Compressor
-1 Fairy Drop
+1 Exp Share
+2 Max Potion
-2 DCE
+1 Fairy Energy
+1 Wonder Energy
If I could fit a 61st card in, it would be a fourth Fairy Garden - being as Diancie relies on a stadium being in play, plus getting a bad opener out of the active early - but I'm hesitant to make any other cuts. So many of the early-game losses came down to not hitting hard enough early, or losing a crucial Remoraid/Spritzee early where Diancie might have soaked the damage.