How to make a unit to cast a spell on low hp?

[HolyWillRise]

Hello. How can i make a unit to cast a spell on low hp?
For example:
Casting Storm wind and earth at 10% HP

And 1 more question. I put mana shield spell to a Boss but he dont use it, why? I tried with unit spell, with hero spell, and make it permamently active and all of those just dont work.. It worked at the start, but now it dont..

 

[Rigborn]

You just order that unit to cast that spell. if you have dds in your map you can use damage event or you can periodically check if it's hp is less than or equal to %10 of his max hp.