Ok, I substituted Grachomp's rocks with Ferrothorn, I was also able to last longer with him, I also switched out Registeel with Bisharp, Scolipede stays in but he is now offensive and Sigilyph got replaced by Mega Latias.
Here is the team:
Ferrothorn (F) @ Rocky Helmet
Ability: Iron Barbs
EVs: 4 HP / 252 Def / 252 SpD
Sassy Nature
IVs: 0 Spe
- Leech Seed
- Toxic
- Gyro Ball
- Stealth Rock
Bisharp (M) @ Black Glasses
Ability: Defiant
EVs: 252 Atk / 52 SpD / 204 Spe
Jolly Nature
- Sucker Punch
- Iron Head
- Knock Off
- Swords Dance
Scolipede @ Black Sludge
Ability: Speed Boost
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Swords Dance
- Megahorn
- Poison Jab
Tornadus-Therian (M) @ Assault Vest
Ability: Regenerator
EVs: 96 HP / 160 SpA / 252 Spe
Timid Nature
- Hurricane
- Superpower
- Knock Off
- U-turn
Starmie @ Leftovers
Ability: Natural Cure
EVs: 248 HP / 32 Def / 4 SpA / 224 Spe
Timid Nature
- Scald
- Reflect Type
- Rapid Spin
- Recover
Latias-Mega (F) @ Latiasite
Ability: Levitate
EVs: 248 HP / 252 Def / 8 SpA
Bold Nature
- Stored Power
- Calm Mind
- Energy Ball
- Recover
The general rule of thumb for dealing with defensive Pokemon spreads is to max out HP first before moving on to the defenses to optimize overall bulk. Your Ferrothorn doesn't really accomplish this. Also, this team is pretty susceptible to Talonflame, so I'd have Heatran or Therian Landorus as at least something that can deal with it:
-Lava Plume
-Toxic
-Taunt/Protect/Roar
-Stealth Rock
Nature: Calm
EVs: 248 HP/192 SDef/68 Spe
Item: Leftovers
or
-Fire Blast/Magma Storm
-Flash Cannon/Hidden Power (Ice)/Stone Edge
-Earth Power
-Stealth Rock
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Air Balloon
Therian Landorus:
-Stealth Rock
-Earthquake
-U-turn
-Stone Edge/Knock Off/Rock Slide
Nature: Impish
EVs: 252 HP/240 Def/8 SDef/8 Spe
Item: Leftovers/Rocky Helmet
or
-Stealth Rock
-Earthquake
-Stone Edge/Knock Off
-U-turn/Explosion
Nature: Adamant/Jolly
EVs: 72 HP/252 Atk/184 Spe
Item: Soft Sand/Earth Plate/Focus Sash
But if you're set on keeping Ferrothorn, here's a better take on it:
-Stealth Rock
-Leech Seed
-Power Whip/Protect
-Gyro Ball/Thunder Wave
Nature: Relaxed
EVs: 252 HP/88 Def/168 SDef
Item: Leftovers
Abiilty: Iron Barbs
I don't see the point of Energy Ball on Latias, when Dragon Pulse can hit Dark Pokemon for at least neutral damage as well for a bit more power. Outside of dealing with the occasional Quagsire, Gastrodon and Mega Swampert, Energy Ball isn't a good coverage option outside of being a STAB move for Grass Pokemon, And as an FYI, Mega Latias fits better on defensive teams; this doesn't appear to be one. I'd advise switching your Mega back to Medicham:
-Calm Mind
-Stored Power/Psyshock
-Substitute/Reflect Type/Dragon Pulse
-Roost
Nature: Bold
EVs: 248 HP/84 Def/176 Spe
Item: Latiasite