^lol that huge double post
If you're going to use Garchomp, you'll wanna have it use Fire Fang or Stone Edge instead of Crunch. Crunch offers Garchomp no extra coverage. Fire Fang and Stone Edge do. And like Platinum said, Mega Garchomp is better on dedicated sand teams where Sand Force is useful. For other items, Yache/Haban/Roseli Berries or Choice Band/Scarf are options.
Your team isn't terribly weak to entry hazards, so a spinner isn't vital. But if you do keep a spinner on the team, I wouldn't go with Excadrill. If you replace Porygon Z with one of the physical attackers Platinum suggested (which is probably a good idea) and add Excadrill to the team, you'll be overloaded with physical attackers and your team will shift from having special wall issues to having physical wall issues. It's also somewhat redundant with Garchomp on the team (as is Starmie with Greninja).
Since you're not laying down your own pointy objects, Defog is an option for you. Maybe Mandibuzz or Crobat?
-Mandibuzz w/Leftovers
Nature: Bold/Impish
Ability: Overcoat
EVs: 252 HP, 252 Defense, 4 Sp.Defense
Moves: Foul Play, Roost, Defog, Toxic/Knock Off
-Crobat w/Black Sludge
Nature: Jolly
Ability: Infiltrator
EVs: 252 Attack, 252 Speed, 4 Defense
Moves: Brave Bird, Defog, Roost, U-Turn
And even if those 2 aren't an option/not appealing, plenty of stuff learns Defog, so maybe something else will work....
Other Defoggers are
[email protected], Scizor and Skarmory. Crobat also uses some bulk investment to let it come in and out more easily (of course, max Attack/max Speed is viable too). Mandibuzz uses 248 HP EVs to make its HP indivisible by 4, making it die to 5 Stealth Rock switch-ins from full health as opposed to 4:
Latias:
-Draco Meteor
-Psyshock/Thunderbolt
-Defog
-Roost/Recover/Healing Wish
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Latios:
-Draco Meteor
-Psyshock
-Earthquake/Surf/Hidden Power (Fire)
-Roost/Recover/Defog
Nature: Hasty/Timid
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
Skarmory:
-Roost
-Whirlwind
-Stealth Rock/Spikes/Brave Bird
-Defog/Brave Bird
Nature: Bold/Impish
EVs: 252 HP/252 Def/4 SDef or 252 HP/16 Def/240 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
Crobat:
-Brave Bird
-Defog
-Roost
-Taunt/Toxic
Nature: Jolly
EVs: 168 HP/140 Atk/200 Spe
Item: Black Sludge
Ability: Infiltrator