2 questions

1.How do you add Chaos/Divine armor to a unit?
2.Is it possible through to order a unit to cast a custom spell (if unit has that spell as a ability)?

For #1:
To add Divine Armor to a unit go to the Object Editor.

Then select the unit you want to add Divine Armor to and scroll down until you see "Comabt - Defense Type", change it to Divine.

For #2:
The Action for a unit to cast a spell, depends on the spell.

If the unit casts an AoE spell you would use "Unit - Issue Order Targeting a Point".
If the unit casts a spell on a target unit you would use "Unit - Issue Order Targeting a Unit".

I think if you want to cast a spell based off of Blizzard, you'll still need to tell the unit to cast Blizzard in the trigger. It'll just cast that spell that's based off of the spell.

:\ I have a similer problem to question 2, tho I want to cast the same
ability of 'event response - ability being cast'.

Here is a link to the thread:
Trigger Casting Question

tnx in advance for any help!

2: yes it is possible. If you look at your custom spell u see an order string, like if u base your custom spell on flame strike the order string will be "flamestrike". If u want to use your custom spell, just tell the unit to cast "human bloodmage - flamestrike".