Scolipede > Ninjask. Its defensive typing is less crippling to get Baton Passing done and it's also bulkier:
-Substitute
-Protect
-Baton Pass
-Swords Dance/Iron Defense/Megahorn
Nature: Timid/Jolly
EVs: 248 HP/20 Def/240 Spe
Item: Mental Herb
Ability: Speed Boost
There are better Pokemon than Weezing out there. If you want to stop setup sweepers, use Mandibuzz, Skarmory (both learn Whirlwind to force out stat-uppers) or Amoonguss (learns Clear Smog as an offensive form of Haze), but take note that the latter shares weaknesses with Scolipede:
Mandibuzz:
-Foul Play
-Roost
-Taunt/Whirlwind
-Knock Off/Toxic/Defog
Nature: Impish/Bold
EVs: 248 HP/136 Def/108 SDef/16 Spe
Item: Leftovers
Abiilty: Overcoat
Skarmory (this is a Showdown! team, at least; Defog Skarmory is pretty hard to get a hold of in cartridges):
-Roost
-Defog
-Whirlwind/Taunt/Brave Bird
-Toxic/Stealth Rock/Counter/Brave Bird
Nature: Bold/Impish
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
Amoonguss:
-Spore
-Giga Drain
-Clear Smog/Sludge Bomb
-Hidden Power (Fire/Ice)/Sludge Bomb
Nature: Bold
EVs: 252 HP/228 Def/28 SDef
Item: Black Sludge
Ability: Regenerator
Light Screen is a subpar option on Blissey; its special bulk is great already. Its pre-evolution, Chansey, is better in most cases, as it has better bulk factoring in Eviolite. However, both are better suited to stall teams and this team isn't one. I think Sylveon is a better candidate, as it packs some form of offensive presence in Pixilated Hyper Voices (Pixilate turns all Normal moves into Fairy ones and powers them up by 30%):
-Wish
-Protect
-Hyper Voice
-Heal Bell
Nature: Bold
EVs: 252 HP/216 Def/40 SDef
Item: Leftovers
Ability: Pixilate
Mega Alakazam can get away with a Modest nature because with Modest, it still outspeeds what it needs to outspeed, such as Greninja and Theiran Tornadus. That, and Timid Mega Alakazam hits slightly less harder than Timid Life Orb regular Alakazam.
Excadrill > Sandslash, simply because Sandslash is slower and weaker than Excadrill:
Excadrill:
-Earthquake
-Iron Head
-Rapid Spin
-Rock Slide/Stealth Rock
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Leftovers/Air Balloon
Ability: Mold Breaker
or
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe (Choice Scarf) or 120 HP/136 Atk/252 SDef (Assault Vest; Adamant only)
Item: Choice Scarf/Assault Vest
Ability: Mold Breaker