You have a huge weakness to Fighting types. And we need to remedy that by removing Terrakion (sorry) and replace it with a Fighting resist. Slowbro
does this well, and can give you A) another Special Attacker, and B) can catch people off guard with Choice Specs. Access to Regenerator also remedies the Sand's damage to it.
Slowbro @ Choice Specs
EVs: 180 HP / 252 SAtk / 76 Spd
IVs: 0 Atk
- Fire Blast
- Grass Knot
- Ice Beam / Psyshock
Ice Beam is the better choice to hit Dragons hard. Also, you could use T-Tar or Stoutland to kill Blissey and Chansey. The investment in speed allows Slowbro to outrun uninvested Gastrodon.