So...
Celebi
Milotic
Forretress
Heatran
Gyarados
Rotom
(This holds multiple resists to 16 types, but lacks a stealth rock resist)
OR
Milotic
Gyarados
Hippowdon
Forretress
Rotom
Heatran
(This holds multiple resists to 16 types, and includes a stealth rock resist, but also raises Sandstorm)
Celebi
Milotic
Forretress
Heatran
Gyarados
Rotom
(This holds multiple resists to 16 types, but lacks a stealth rock resist)
OR
Milotic
Gyarados
Hippowdon
Forretress
Rotom
Heatran
(This holds multiple resists to 16 types, and includes a stealth rock resist, but also raises Sandstorm)