A Calculator is like some kind of uber-mage. It can use most spells from the Wizard, Priest, Time Mage and Oracle classes in combination with Math Skill. Math Skill multiplies a value (experience, height, level or CT) with a number (prime numbers, 3, 4 or 5), and every unit that meets the requirements becomes a target. It's usually a bit sloppy (often both enemies and allies will be targeted, or not all the units you want will be targeted), so to prevent that you have to be very careful about height placement, or else be cheap and give your units Chameleon Robes while blasting everyone with Holy. =P Calculators themselves have crappy stats, so once you've mastered it it's best to give Math Skill to someone with better magic.