Elementary Medicine is an achievement that requires the player to cast an elemental spell on an elemental wizard in order to heal him. Spells from the Ancient Magicks spellbook also work.
The wizards can be found north-west of Port Sarim, alongside Malignius Mortifer. Whenever they are attacked by a spell of their corresponding element, they are healed instead of damaged, so attacking the air wizard with an air spell will heal him. Using a powered staff (ie. Staff of Air), will not count.