You do not have to do all of these at the plot related decisions as stated, as you will still unlock the achievement after defeating the Wizard if your corruption is 0%, but this is how I did it:
Give Spree’s Food
Do not burn down the Sacred Grove in Evernight Forest
Choose Rose over Velvet
Save the elves from the slugs in the Glittering Mine
Save the elven women rather than taking Goldo’s Gold in Goldo’s
Stronghold
Give back the Mother Goddess Statue
Spare the traitors at Spree ((Occurs after Kahn has trashed Spree) Return to the tower to spare the traitors.)
Along with not killing any civilians or plundering any houses as this will add corruption, however performing the "good" deeds above will reduce your corruption by 2%.