Probably just causal wifi singles. He won't be perfectly bred but I like him and wanna see how he works
3v3 or 6v6? Battle Spot singles is only 3v3, which I'm not a big fan of.
If its 6v6, I'd roll something like:
Sylveon@Leftovers
Cute Charm
Modest
252 HP / 60 Def / 196 SpAtt
-Moonblast
-Heal Bell / Toxic
-Wish
-Protect
Pixelate is weird since Sylveon is a Special Attacker. You can flip the 196 SpAtt EVs into Sylveon's other stats as desired, but the 60 Def EVs and 252 Evs are to force Garchomp Earthquake's to 3HKO Sylveon with Leftovers recovery. Wish is for team support and self recovery, as is Heal Bell. Flip Heal Bell for Toxic if needed to break certain walls. Moonblast for decent damage.
Not super impressed with Sylveon's movepool though.