I'd think about psyshock, it lets you beat Mega Venusaur (who can otherwise wall you) and other sylveons well enough, and being mixed (technically) could never hurt.
Actually.. that flower dinosaur has been pretty annoying with its Seed/Synthesis set ._. As for other Sylveons, against Defensive Sylveons (who tend to run Bold nature mostly with 252 Def), Hyper Voice deals more damage.
+1 252+ SpA Pixilate Sylveon Hyper Voice vs. 252 HP / 4 SpD Sylveon: 222-262 (56.3 - 66.4%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ SpA Sylveon Psyshock vs. 252 HP / 252+ Def Sylveon: 120-142 (30.4 - 36%) -- guaranteed 4HKO after Leftovers recovery
Psyshock is interesting though, does it do anything but hit Poison-types though?