How to make a unit to cast a spell when beign attacked?

[HolyWillRise]

I am making a boss which use mana shield but theres something broken and it deactive mana shield.. So i need a trigger where when the boss beign attacked to cast mana shield which cant be deactivated.

Also how can i make a trigger where the Boss (a unit) cast a spell at low hp? I use the picture below but it doesnt work.

 

[Warseeker]

I am making a boss which use mana shield but theres something broken and it deactive mana shield.. So i need a trigger where when the boss beign attacked to cast mana shield which cant be deactivated.

• Trigger
  • Events
    • Unit - A unit is Attacked
  • Conditions
    • (Triggering unit) Equal to YourUnit
  • Actions
    • Unit - Order YourUnit to Neutral Naga Sea Witch - Activate Mana Shield

Also how can i make a trigger where the Boss (a unit) cast a spell at low hp? I use the picture below but it doesnt work.

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