Since its a skill, you have to time it to when he's going to die like you would for Karma in order to keep her alive for the longest amount of time possible.
Its only something you don't need to press a button for if its a unit type perk or if its an actual passive. Ex. The Devil/Genie unit that is over on jp side of things has a class type perk that makes her just return to the unit selection bar upon 'death' but it won't count as one for the purposes of 3*ing a map.