Hi Owen, nice team
The first thing that popped out at me is the fact that this team is both beaten by and would desperately love to use, hazards. I'm gonna advocate putting Forretress over Slowbro. I feel like this is the best way to go for the team because without Chansey then Tornadus-T can just tear up this team. Aside from that, I don't really think you'd need another Water Resist since there's Amoongus and Gastrodon, while hazards will take care of U-Turning teams. Forretress also gives you a much needed Steel resist as something like SD Outrage Chomp and hazards could be a huge problem for this team. Silverlight isn't loading but I guarentee you that SD YacheChomp would 2hko or [probably OHKO] everything with a +2 Outrage if it has spikes support. Forretress can provide Spikes to support the rest of the team also, so you can threaten strong attackers more. Aside from the much-needed switch to Forretress, I'm seeing two things as problems, one is Drought teams and the other is SD Roost Scizor. Drought Teams can possibly overwhelm Blissey with something like a trapping Heatran or even Volcarona, and Hippowdon is going to have a hard time winning the weather war if it can't take attacks from Pokemon like Heatran or Ninetales when it has to change the weather [again, wish the calc was working lol]. As for SD Scizor I feel like it could have a hard time since there are 2 Pokemon with Scald, but late-game it could be a menace if some of your Pokemon are gone. Additionally, Slowbro can OHKO but it has to take a +2 Bug Bite first, so you could be in trouble if you needed Slowbro for another Pokemon [even with Regenerator], so a cunning opponent could easily employ this strategy against you. Anyways I suggest going for a full stall spread and swapping out Terrakion for a Gliscor. Terrakion is obviously great but I feel like its setup bait for Scizor for one thing and doesn't have the support of any other sweepers to weaken its counters, so a full stall team seems superior as Uomo Solo said. Gliscor easily takes on SD Scizor with Taunt and also beats stuff like Terrakion, Dragons, etc. In addition to this, it can act as a status absorber, making Breloom immediately less of a problem. Because of this fact, I also advocate putting Wish>Aromatherapy on Chansey, so that Forretress can have a lot better staying power and not falter against teams that have Ferrothorn or something, or keep it alive against Volt Turn when it has to take their Vswitches/Uturns.
Forretress (M) @ Leftovers
Trait: Sturdy
EVs: 252 HP / 176 Def / 80 SDef
Relaxed Nature (+Def, -Spd)
-Spikes
-Gyro Ball [0 Speed IV]
-Rapid Spin
-Volt Switch
Gliscor (F) @ Toxic Orb
Trait: Poison Heal
EVs: 252 HP / 188 Def / 68 Spd
Impish Nature (+Def, -SAtk)
- Protect
- Earthquake
- Ice Fang
- Taunt
Change Summary:
Hippowdon: 252 HP / 244 Def / 12 SDef, Impish -> 252 HP / 4 Def / 252 SDef, Careful
Chansey: Aromatherapy->Wish
I think this will be a really good team with the changes, I'd even use it myself ;)