When it comes to Moves, cruising with Sylveon was very much a case of being content with what you have and making the most of it. The Fairy Eevee has a tiny offensive learnset by default, with only twelve offensive Moves available through TM and nine by leveling up. The latter include four Special Fairy Moves of various power, a perfect setting for Sylveon given the combination of his high Sp. Attack and the STAB. But wait, there's a catch — and a massive one at that. See, Sylveon learns three of those Fairy Moves before lv. 20; yet in X&Y, Eevees can only be recruited right after the second Gym, at lv. 20-22. Heck, I think we all see the big issue here: in a regular X&Y run with no breeding involved, you're gonna miss on Sylveon's first three Fairy Moves no matter what; and by the time you reach the Move Reminder, you won't have any use for these Moves anymore because of their low power. GameFreak visibly spotted the problem and tried to mitigate it in Gen VII, by making Sylveon learn Fairy Wind upon evolving; you still miss out on Disarming Voice and Draining Kiss in the process, but I guess that's better than nothing.
Getting deprived of three quarters of Sylveon's Fairy Moves is already galling enough, but that's not even the end of Sylveon's learnset troubles. The Fairy fox-meets-rabbit suffers from what I dubbed 'The Great Move Gap': he doesn't learn any offensive Moves by leveling up between lv. 20 and lv. 37. Add to this the fact that the offensive TMs he can learn appear very late in the game, and you get the situation I wound up in during my run: until my Sylveon reached lv. 37, I was stuck with the base offensive Move pool he wielded as an Eevee — namely Quick Attack, Bite and Swift. I managed to make do with those Moves pretty neatly, shall I say: Swift as my main choice, Quick Attack to finish foes, and Bite for anything immune to Normal. Still, as I saw levels pile up and no new Moves come my Sylveon's way, the whole thing started to look like GameFreak was taking the piss out of me, and I seriously wondered if we were dealing with some trolling of Pidove proportions there.
But then came the fated lv. 37; and oh dear, was it a turnaround indeed. Moonblast: Fairy Special Move, 95 power — nuff said. My Sylveon was still just as starved of Moves as before, but he suddenly became able to one-shoot everything with abandon. Needless to say, I spent the rest of my run spamming Moonblast. Even after I got hold of Psychock and Shadow Ball in the late stages of the game, Moonblast remainded my go-to Move thanks to its enormous power and the STAB. I really cherished that Move, because of how long I had to wait before I got hold of it and how much it changed my fighting fortunes. But I cherished even more Swift, which remained in my Move pool during the whole game. As a multi-target Special Move with 60 power, it remained relevant all the way to the Pokemon league, and I always found good uses for it. Fun fact: in french, that Move is called 'Meteors', which I deem ten times more evocative than Swift — on top on hinting nicely at the multi-targeting capability of that Move.
In the end, Sylveon exceeded all my expectations. This is not the most popular eeveelution of them all, and I half-feared that he would let me down on the battlefield; but despite his tiny learnset, my Sylveon did a great job on that front and conquered the Pokemon League without breaking a sweat. See you soon for more Pokemon goodness, dear fellow gamers! Until then, as usual: thanks for reading, and drop by anytime!