https://www.aonprd.com/SpellDisplay.aspx?ItemName=Sacred%20Nimbus
When the opponent takes damage: when it attacks you, or when it hits you and deals damage?
When they hit you so if they miss they are fine but if they hit they get hit back.
Agreed. The relevant line:
Any evil creature striking you with unarmed strikes, natural weapons, or a handheld weapon deals normal damage, but at the same time, the attacker takes 1d6 points of damage + 1 point per caster level (maximum +15).
"[S]triking you" and "deals normal damage" indicate a hit. Note that this is the same language used in fire shield, the arcane spell sacred nimbus was undoubtedly based upon, and it is also an "on hit" effect.