(information derived from sonny.wikia.com unless stated else) Note that cooldown is effectively one less for AI than for Sonny.

Enemies that can heal:

ZPCI Medic: (Stage 7)

• has a 50% chance to heal allies between 90% and 80% health
• always heals allies with less than 80% health You however can outright kill him so it doesn't matter

Ghost Samurai: (Stage 10,15,16)

• Healing Spells: SWAT Heal (self or ally, CD: 6)
• 50% chance to heal allies between 50% and 30% health
• always heals allies with less than 30% health

ZPCI Hunter: (Stage 12,14)

• Healing Spells: SWAT Heal (self or ally, CD: 6)
• has a 50% chance to heal allies between 85% and 60% health
• always heals allies with less than 60% health

Sensei Ishiguro: (Stage 17)

• Healing Spells: SWAT Heal (self or ally, CD: 6)
• has a 50% chance to self-heal if between 95% and 75% health
• always self-heals if below 75% health

Shaman Controller: (Stage 19)

• Healing Spells: SWAT Heal (self or ally, CD: 6), Water Heal (self or ally, CD: 0), Water Regen (self or ally, CD: 4)
• has a 50% chance to heal allies between 90% and 80% health
• always heals allies with less than 80% health

Shaman of Blades: (Stage 18,27,28)

• Healing Spells: Cell Reformation Shaman (self, CD: 18)
• has a 50% chance to heal between 75% and 40% health
• always heal below 40% health

Shaman of Death: (Stage 21,27)

• Healing Spells: Heroic Motivation (self or ally, CD: 3, dispels one debuff)
• has a 30% chance to heal allies between 60% and 30% health
• always heals allies with less than 30% health

Shaman of Life: (Stage 24,28)

• Healing Spells: SWAT Heal (self or ally, CD: 6), Water Heal (self or ally, CD: 0), Water Regen (self or ally, CD: 4)
• has a 70% chance to heal allies between 85% and 75% health
• always heals allies with less than 75% health

Shaman Survivor: (Stage 30)

• Healing Spells: Reform (self, CD: 8 )
• has a 50% chance to either self-heal or re-energize if between 80% and 60% health
• always either self-heals or re-energizes if below 60% health

Totem: (Stage 21,24,30)

• Healing Spells: Heroic Motivation (self or ally, CD: 3, dispels one debuff)
• has a 70% chance to heal allies between 95% and 75% health
• always heals allies with less than 75% health I have seen Totems that healed themselves when an ally was low. Maybe I will look into that later.

ZPCI Elite: (Stage 26,32,34)

• Healing Spells: SWAT Heal (self or ally, CD: 6)
• has a 50% chance to heal allies between 80% and 60% health
• always heals allies with less than 60% health

Elite Medic: (Stage 34)

• Healing Spells: 3x SWAT Heal (self or ally, CD: 6), Water Heal (self or ally, CD: 0)
• has a 50% chance to heal allies between 80% and 60% health
• always heals allies with less than 60% health

Dokebi the Striker: (Stage 29)

• Healing Spells: Reform (self, CD: 8 )
• has a 50% chance to either self-heal or re-energize if ally is between 75% and 40% health
• always either self-heals or re-energizes if ally is below 40% health

Vendetta the Breaker: (Stage 29)

• Healing Spells: Heroic Motivation (self or ally, CD: 3, dispels one debuff)
• has a 30% chance to heal allies between 60% and 30% health
• always heals allies with less than 30% health

Argalla the Mender: (Stage 29)

• Healing Spells: SWAT Heal (self or ally, CD: 6), Water Heal (self or ally, CD: 0), Water Regen (self or ally, CD: 4)
• has a 70% chance to heal allies between 85% and 75% health
• always heals allies with less than 75% health

Galiant the Paladin: (Stage 36)

• Healing Spells: Paladin Heal (self or ally, CD: 0), Pally Intervention (self or ally, CD: 24, shields 440 damage)
• has a 50% chance to either heal or shield himself if between 50% and 40% health
• always either heals or shields himself if below 40% health

Enemies that can use other "defensive" skills:

Frost Zombie: (Stage 8,13)

• Defensive Skills: Suppression (70% damage reduction, CD: 5)
• has a 50% chance to use Suppression if between 35% and 25% health
• always uses Suppression if below 25% health

Ghost Assassin: (Stage 11,15)

• Defensive Skills: Shadow Blend (+500% Speed, CD: 15)
• has a 50% chance to use Shadow Blend if between 50% and 45% health
• always uses Shadow Blend if below 45% health

Baron Brixius: (Stage 35)

• Defensive Skills: Shadow Blend (+500% Speed, CD: 15)
• has a 50% chance to use Shadow Blend if between 50% and 30% health
• always uses Shadow Blend if below 30% health

Rockstar: (Stage 23)

• Defensive Skills: Suppression (70% damage reduction, CD: 5), Iron Skin (+150 Health, increases phys. resistance, CD: 7)
• has a 50% chance to use either Suppression or Iron Skin if between 30% and 25% health
• always uses either Suppression or Iron Skin if below 25% health

Aside from that, skills are mostly chosen at random. So there is no reliable way to prevent an enemy from using stuns. The only thing you can try is to make them stun Veradux instead by having more Health than him.

Inconsistencies in AI-behavior:

Sometimes the AI behaves differently from what it is supposed to. This is mostly exclusive to stages with multiple opponents:

• sometimes AI heals themselves instead of their (weakened) ally. Interestingly enough, this can also happen with units that cannot actually heal their allies (such as shamans of blades).

• the AI "knows" when a mate heals an ally and considers this for their own AI. This means, you will seldom see 2 units healing the same target on the same turn. My theory is that this happens on a "priority" base. So first, the game decides what "unit 1" does. Then it decides the action of unit 2, based on the action of unit 1. This would also explain, why paladin always never heals at the begin of the fight but starts healing more frequently at the end of the battle (because veradux is either dead or out of focus)

• on multi-opponent stages there is a reduced chance the AI uses defensive skills at all. This however seem to vary depending on stage-to-stage basis. Also, I've witnessed multiple times, that on certain stages, enemies don't seem to use their defensive skills at all until only one of them remains. Examples for this would be the last two stages before Sensei as well as the fight against the two Frost Zombies.