Divert: " This unit cannot lose its last HP unless it is already at 1 HP prior to taking damage."
Any damage dealt to a unit within an engage is considered a single instance of damage. Therefore, it is not possible to defeat an enemy with Divert and more than 1 HP with a single engage, no matter how many different damage-dealing skills you use. You must get it down to 1 HP, then use a new instance of damage (a new engage, a class ability, an item, etc) to be able to defeat it.