What's better %INT, %Damage or %Mattk?

Elemental Amplification doesn't add magic attack. It's more like %dmg, no idea if they stack additively though.

AMP is a total damage multiplier and it is always calculated separately. (In other words, it doesn't add to a %Damage potential line. %Damage gets applied after AMP is already calculated.)

Bishops do not get Amp. This is why they get hosed. Bish skills have similar damage range as a mage, but mages get Amp which makes them far better. Bishies get Holy Focus, but it's a pathetic equivalent.