Ability: Rough Skin
Item: Leftovers/ Yache Berry/ Choice Scarf
EVs: 252 Atk, 252 Speed, 4 HP
Swords Dance/ Dragon Claw (Scarf set)
I prefer this set for Garchomp. Draco Plate is useless imo since Garchomp can either benefit from a Scarf set, or use Swords Dance with leftovers.
Ability: Flame Body
Item: Leftovers/ Life Orb
EVs: 200 HP, 252 Def, 56 Speed
Roost/ Giga Drain
Bulky Volcarona. Flame plate is useless because it only powers up one move, and you already have Quiver Dance so if you wanted something that powered up moves even more, use Life Orb. Also, Leftovers for Roost, Giga Drain for Life Orb.
Ability: Moxie/ Shed Skin
Item: Lum Berry/ Life Orb
EVs: 252 Atk, 252 Speed, 4 Sp. Def
Why the plates? Seriously I don't get why people put plates on anything other than Arceus; they only power up one move, and only by 20%. If you really wanted that, then use Expert Belt. At least it'd power up all super-effective moves.
Shed Skin is an option because if you get burned, your Dragon Dance set up is ruined. And Zen Headbutt to deal with those fighting types.
Ability: Natural Cure
EVs: 252 Def, 252 Sp. Def, 4 HP
Flamethrower/ Ice beam/ Seismic Toss
Toxic/ Thunder wave
If you're using Serene Grace, you might as well put a a move that has a 20%+ chance of doing something actually useful. Psychic will only lower the opponent's special defence, coupled with no STAB and Blissey's barely acceptable special attack, it does not take advantage of Serene Grace at all. And nor are you investing any EVs in Blissey's special attack to warrant a Serene Grace set. You need to consider things better.
Also, Counter: if Blissey gets hit by a physical move, it's likely to OHKO her, making this move useless.
EVs: 252 Def, 252 HP, 4 Sp. def
Flame Thrower/ Ice Beam
Slowbro > Slowking, because you have Blissey on your team to wall special moves, and now you need a physical wall, especially one that can deal with moves like Hi-Jump Kick.
Item: Leftovers/ Shed Shell (to ensure escape so you don't lose your spiker)
EVs: 252 HP, 252 Sp. Def/ Def, 4 Speed
Stealth Rock/ Spikes
Basic set for Skarmory, not much to say; though Skarmory is susceptible to taunt with your current set. Having said that, usually when you taunt you will simply switch out for an attacker.
Making Skarmory physically or specially defensive is your choice, since you already have Blissey and Slowbro.