Farming with SSF firelock.

Wondering what the best act to farm a firelock would be. I know act 1 and 4 have tons of fire immune and i feel like act 5 has bad mob density outside of WSK. It seems like it would be 2 or 3. I don't have a way to sunder fire immune yet, outside of a lower resist wand anyway and being single player offline I might never find what a need for that.