Vertical Drop Heroes HD

Before you attempt this achievement, head to the main menu and go into Options, then turn off Auto Attack. Otherwise if you are within striking distance of an enemy, your character will attack an enemy thus removing all Pacifist Orbs in a level.

The orbs are blue in colour and are frequently dotted about the levels. You really can't miss them. Just collect your 12 and the achievement will unlock. But be careful, as you can easily get swarmed by enemies. Best to unlock this on the first level.