cotton guard or light screen?
Why? Ampharos has good bulk and Cotton Guard can make it annoying af against physical attackers. A good set: Ampharos @ Ampharosite EVs: [180 Hp/200 Sp.atk/128 Def] / [180 Sp.atk/100 Def/128 Hp/100 Sp.def] Nature: Modest Ability: Static ---> Mold Breaker -Cotton Guard -Thunderbolt -Dragon Pulse -Hidden Power [Water]/Hidden Power [Grass] This set works amazing. Modest because this thing destroys at base 165 sp.atk. This is meant to take physical hits but works better under Light Screen support. Hope I helped! -It's a secret!
Honestly, with Ampharos's high defenses, I don't think you need a defensive move at all, but you should rather use Agility. This is the moveset I'd use:
Ampharos @ Ampharosite Ability: Static -> Mold Breaker Modest Nature 252 SpA / 252 Spe / 4 HP -Agility -Dragon Pulse -Thunderbolt -Hidden Power Fire