Need help creating a guard spell

[AbsoIuteZero]

I'm trying to create a spell that works similarly to spirit link, just one way and not both. Essentially what I want the spell to do would be for a tank-like hero to use the spell "Guard" on a nearby friendly unit and the tank would take 50% of the targeted unit's damage for 10 seconds. The only stipulation would be that if during that time the tank is guarding the targeted unit the tank is attacked, I WOULD NOT want the guarded unit to take 50% of the damage that the tank is receiving - I want the tank to take full damage. I've kind of come to brick wall here and am in need of some direction. Any help would be very much appreciated.

 

[Jagan]

You'll need a trigger to check when a unit with the buff from the Guard ability takes damage before calculating the damage taken/2 and then finally applying to the caster of the Guard ability.

 

[AbsoIuteZero]

I guess I should have been a little bit more specific with my post. I understand how it should work, but knowing and doing are a bit different, and as far as the doing part I am quite incompetent. I hate to be a bother, but could you show me how that might work?