The heal can be replicated by using the totem of life, or by eating food.
griffins also upgrade to become mobile healers, and dryads self heal near trees.

The spell ingredient valerian can produce the spells:

raise dead,

summon wolf and

heal.

The wolf is a standard troop, and raise dead you to be constantly on top of the action to be effective, but heal is the most useful spell here. Heal keeps your troops healthy, more experienced, and therefore more likely to upgrade.