One of my favorite counters to Serperior right now is Goodra because of Sap Sipper, you should really consider adding it to your team. It wouldn't be recommended to have 2 dragon types, so consider switching it for Latios.
Try this set for Goodra:
Goodra @ Assault Vest
Ability: Sap Sipper
EVs: 240 HP / 252 SpA / 16 Spe
Modest Nature
- Draco Meteor
- Fire Blast
- Sludge Bomb
- Rock Slide
Like some others said, try out Rotom-W if you find that Lanturn doesn't do much for this team.
By the way, what other Pokemon are your biggest weaknesses?
Some other options to consider are Krookodile and Excadrill. Excadrill could be a great spinner in this team. Personally I've always thought Tyranitar had too many weaknesses especially in 6th gen, but back in 5th gen it was prob the best Pokemon out there. I just don't look into using it much anymore, because there are better Pokemon out there.
But yeah, try out Krookodile with Earth Plate or Dread Plate. It's still a strong Pokemon despite it being UU and could be a good substitute for Tyranitar.
I hope this helps a bit!
Muddy Water or Earthquake is preferred over Rock Slide on Assault Vest Goodra because right now it's walled by Heatran. Talonflame and Charizard are already hit hard by Draco Meteor