You want your Sylveon's EV spread to be 252 HP, 244 Defense, 12 Speed. Something about outspeeding a few threats.
In addition to MegaSaur, you can also use Trevenant, Gourgeist, or Tangrowth for defensive Grass types (if you go that way):
-Super Size Me Gourgeist w/Leftovers
Nature: Impish
Ability: Frisk
EVs: 252 HP, 252 Defense, 4 Sp.Defense
Moves: Will-o-Wisp, Leech Seed, Seed Bomb, Shadow Sneak
-Trevenant w/Sitrus Berry
Nature: Adamant/Impish
Ability: Harvest
EVs: 252 HP, 184 Defense, 76 Speed
Moves: Will-o-Wisp, Leech Seed, Protect/Substitute, Phantom Force/Shadow Claw
-Tangrowth w/Leftovers
Nature: Bold
Ability: Regenerator
EVs: 252 HP, 252 Defense, 4 Sp.Defense
Moves: Giga Drain, Substitute, Leech Seed, Hidden Power Fire/Rock
Something like that.
Top threats in OU will eat Trevenant alive (Bisharp comes in on Sub-less Trevenant and cripple SubSeed variants with Knock Off, the Megazards and Mega Pinsir will use it as setup bait, etc.). Just putting that out there. I don't think it's that viable anymore, even though I enjoyed using it for a time. While Gourgeist suffers from the same problems, it has better bulk to make use of. It also can't learn Shadow Claw:
-Will-o-Wisp
-Leech Seed
-Seed Bomb
-Protect
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Frisk
Tangrowth is too slow for SubSeed:
-Giga Drain/Leaf Storm
-Knock Off
-Earthquake
-Hidden Power (Ice/Fire)/Rock Slide
Nature: Sassy
EVs: 252 HP/4 Def/252 SDef
Item: Assault Vest
Ability: Regenerator
or
-Giga Drain/Leaf Storm
-Knock Off
-Sleep Powder/Earthquake
-Hidden Power (Ice/Fire)
Nature: Relaxed
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
Ability: Regenerator