Sweepers are frail by nature.
I'd switch Ferrothorn with Heatran. I assume this is for Battle Spot singles, where its 3v3. While Ferrothorn is good, hazards generally aren't that useful in 3v3s. Heatran allows you to have a Steel type but also gives you a special attacker (yes you have Rotom but you need more firepower). Heatran is also 4x resists Ice, which Garchomp and Gliscor hate, while Gliscor and Rotom-W are immune to its nasty Ground weakness and Rotom also resists Water for it (and Heatran has a 4x resist to Grass for Rotom). Additionally, it has a 4x resistance to the Steel attacks Sylveon hates and is immune to Poison as well, while Sylveon resists Fighting for it. It also resists Fairy for Garchomp. So I think it fits nicely on the team.
-Heatran w/Air Balloon
Nature: Timid
Ability: Flash Fire
EVs: 252 Sp.Attack, 252 Speed, 4 Defense
Moves: Fire Blast, Earth Power, Flash Cannon, Ancient Power/Hidden Power Ice
or
-Heatran w/Leftovers
Nature: Calm
Ability: Flash Fire
EVs: 248 HP, 252 Sp.Defense, 8 Defense
Moves: Lava Plume, Protect, Will-o-Wisp, Roar
If you wanna try Dragonite out, here's a set:
-Dragonite w/ Weakness Policy
Nature: Adamant/Jolly
Ability: Multiscale
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Dragon Claw, Extreme Speed, Dragon Dance, Earthquake/Fire Punch
That still leaves you with one open slot though....not sure what to put in.....
Nah ンン
“No, I... I have to be strong. Everyone expects me to."