Okay so I'm probably going to revamp most of your team
Breloom @ Toxic Orb/Life Orb/ Lum Berry
Trait: Poison Heal/Technician
EVs: 252 Spd / 252 Atk/ 4 HP
- Drain Punch/Mach Punch
- Seed Bomb
- Swords Dance
Okay so since Breloom basically can't take a hit with 60/80/60 defences its probably best to go on the offensive with it. Instead of your original EVs (252 Spd / 124 SAtk / 124 Atk / 8 HP) I made it basically with full attack and full speed. This is because the 124 SAtk evs were useless considering you didn't have a Special Attack, and also your nature (Timid) reduced your attack, which is basically counterproductive considering Breloom is a physical attacker. This Breloom is just pure offensive, spore, setup, and kill. Drain Punch helps you heal but Mach Punch has priority. If you go with Mach Punch, Technician with Life Orb/Lum Berry are better abilities/items as Technician boosts the power of Mach Punch along with STAB.
With your second pokemon Electivire, personally I wouldn't use it - Electivire is completely outclassed in the OU tier by a plethra of other pokemon. I'd probably replace it with this;
Rotom-W @ Choice Scarf
EVs: 252 SAtk, 252 Spe, 4 HP
-Hidden Power Ice/Hidden Power Fire
Rotom-W is an excellent Electric type in place of Electivire, and its ability Levitate gives it an immunity to the bane of Electric types - ground. With a Choice Scarf and a Timid nature it hits 447 speed and becomes a good revenge killer. Off course you can instead go with a Modest nature, allowing it to hit harder and still maintaining a speed of above 400. Volt Switch is a possible option instead of Thunderbolt so you can run away in a bad situation, but of course then you have nothing to really hit water types with. Hydro Pump is self-explanatory, HP Ice/Fire is just a filler, the first hitting Dragon and grass types hard while the second hits pokemon like Ferrothorn harder. Trick is just to screw around with an opponent's wall >:D Also Rotom-W has decent bulk, so you can take an attack with it if neccesary
For your next pokemon, you used Aggron. Aggron has a massive base 180 defence, a strong base 110 attack and a useable 70 HP granted. However his Spd is at a measly base 60 so his not gonna survive alot of those. Also his low speed can be used against him. However he is still useable, so you can continue using your Aggron if your want. If it was up to me, I'd replace Aggron with this:
Bronzong @ Leftovers
IVs: Speed - 0
EVs: 252 HP, 252 Def, 4 SDef
This guy is an excellent alternative Stealth Rock setter. With 0 speed IVs, his gyro ball can hit hard on unresisting Pokemon and Earthquake hits Steel types. Also because of Levitate he only has 1 weakness, being fire attacks so he'll almost definitely get down Stealth Rocks for your team. Toxic is just to add some status to the opponent's team. He can also tank the Dragon type attacks that the rest of your team can't really handle. This guy is officially the Defensive wall of the team.
Next, your Umbreon is good overall but since you already have a defensive wall , you could try making it a more supportive player;
Umbreon @ Leftovers
EVs: 252 HP, 4 Def, 252 SDef
- Foul Play
- Heal Bell/Yawn/Toxic
Foul Play lets you take advantage of the physical attackers which will be looking to exploit Umbreon's lower defense, since Foul Play's damage is calculated from the opponent's attack stat. Wish is to heal and Protect is to cover yourself against the charge turn for Wish. Wish can also be used to give your other Pokemon some life, and protect can work well with Yawn, as it also protects you on Yawn's charge round, and forces to opponent to either switch or have a sleeping Pokemon. Toxic on the other hand, when combined with Wish and Protect and help you rack up damage on the opponent's Pokemon.
Garchomp's great and all but your moveset is a little flawed so;
Garchomp @ Lum Berry
Trait: Rough Skin
EVs: 252 Spd, 252 Atk, 4 HP
Kind of standard offensive Garchomp set. You don't need stealth rock since Bronzong already has it, Swords dance is a good setup move, Fire Fang works instead of Aerial ace - they hit basically the same things and Fire Fang is slightly stronger. Earthquake is for STAB and hits steel, rock types, etc. better than Brick Break. Outrage > Dragon Rush because of higher base power, better accuracy and Lum Berry gives you temporary relief from confusion which occurs after 2/3 turns of Outrage. Also Lum Berry can help garchomp absorb status, so when faced with a pokemon with Will-o-wisp Garchomp can fire off an extra attack or take a turn to setup.
Last Pokemon: of course you can still use Blissey if you want, and I notice you neglected the EVs on Blissey so;
Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 Def, 252 SDef, 4 HP
Since Blissey's speed is really not high enough to abuse substitute and her defense is low, investing fully into defense allows her to take some attacks and her HP is already massive, it doesn't matter too much. Aromatherapy allows Blissey to remove teammate's statuses but if your running toxic orb Breloom it kinda destroys the point of having breloom poisoned. Flamethrower gives it some form of attacking, largely on the steel types which aren't affected by Toxic - but Heatran still walls this set to the ends of the universe. Softboiled is for recovery.
However, regarding your last pokemon, since you already have a defensive synergy in the form of Umbreon and Bronzong, and only 1 special attacker which works more as a revenge killer rather than anything else. This means if you lose Rotom-W and the opponent has a Defensive wall you won't have much to break through. So possibly in place of Blissey I'd suggest;
Latios @ Life Orb
EVs: 252 Spd, 252 SAtk, 4 SDef
- Draco Meteor
Draco Meteor hits like a tank, and your not gonna find alot of pokemon which can survive 2 of those in a row. Surf and Psyshock add coverage while Roost rounds off the set, allowing Latios to heal and also to mitigate the Life Orb damage. Alternatively you can replace Life Orb with Expert Belt to bluff a Choice set, and replace Roost with a 4th attack of choice. Sticking with Latios also lets Breloom use Toxic Orb/Poison Heal as there's no aromatherapy to screw that combo up :D
Also if you want, you can instead replace Umbreon with Latios instead and keep Blissey on your team, but in that case I'd probably change Blissey's set to Wish, Protect, Toxic, Flamethrower so you can heal teammates as well.
Oh god how much did I write. Sorry if your eyes fall out after reading this ;-;. Of course these are all suggestions and you can always stick to your current team ^_^
Hope I helped and Good Luck