There are four types of defensive spells in Royal Revolt 2.
Health is generally considered to be the most important aspect of the hero in order to survive in combat. The Heal spell restores the health of the hero as well as all nearby units during combat. Additionally, the Blessing scroll spawns a healing area that instantly heals the hero and all units that pass through the healing zone.
Shield is another important spell that adds more defense to the hero and all troops within range of the hero. The Shield spell essentially gives your hero and units an additional health bar that lasts until the unit has received enough damage.
Initially Shield was rarely used because of its low protection, but after Update 1.6.0, the Shield spell became much stronger and is now a strong defensive spell for the hero and troops.
Stun is a defensive spell that paralyzes all units and towers within range. It's very powerful against troops and towers and can last many seconds, which allows your hero and units to continue dealing more damage. Additionally Stun effects are very effective against Gargoyles so that you can evacuate the area before they can bomb your units.
There are also monsters with the ability to stun. The Mummy uses this spell quite often when engaging enemies. Ogres can gain this ability through the Stunning Ogre alliance boost. In any case, the hero is never fully stunned but only slowed down.
Regeneration is a special ability that heals units over time such as the Holy Paladin , Frenzy Frost Blaster but also Heal spell. Its benefit is to heal troops for themselves or also to heal nearby troops like Frenzy Frost Blasters do when they die.
Transform is a unique ability that allows a pal to transform into a Beast for few seconds. It's useful during the narrow U turn of a base to destroy or weaken (according to the pal that you choose) many towers and obstacles around at same time. It's useful also during the fight against a defensive beast. Depending on the pal you select, transform can be a very deadly weapon.