I'm not really sure what you mean by set armor to 75%, you mean reduce it to 75% of whatever it was on units that take damage from a basic attack?
I'd have a "dummy ability" that does nothing (only has text etc.) and use
Damage Engine 5.7.1.1 to detect whenever damage is taken from a basic attack from the correct hero, give that unit an ability based of Devotion Aura, check the "Percent Bonus", make it target self only, set the Armor Bonus to -0.25 (unless it has a buff indicated by that ability). The hard part is to remove it after a duration. You will need to attach a unit to a timer and remove it when the timer ticks, and that's not too easy, but are many ways to do it.
If you want only 25% more damage taken, it's waay easier (apply a buff, and have a damage event filter on that buff as condition, and increase damage taken by 25%).
It might be possible to use a "buff system" or something, but I have not used one and cannot help with that.