ADamage shield without buff -> how to?

Hi guys, I am making a spell that simply prevents a unit from dying ... there are no buffs, no bonuses, no shield life, no complications involved -> if the attack damage is greater then the unit's current hp, I simply stop it.

However, I am having problem, I can't use any examples, and my knowledge on the shield system is very reduced yet (only made 1 spell using shield yet).

So, I need to know: when I add a shield to a unit, do I need a Buff on it?
Do I need a function called Damage?

If not, then how can I make the damage prevention without these things?

Any code examples, or general algorithms would be very appreciated.

Wait wait wait... You made a whole shield spell without understanding what you were doing?

Ignore it... Thing is I made another shield spell that simply prevent the units from dying ... but thing is the units WERE dying ... Facing such facts I assumed the problem was in my shield spell ... but now I see it is not on the shield spell ... it is on the critical strike spell ....

Mainly, I guess I know more than I think xD
So basically I make no idea why my units don't prevent damage when I use xedamage ... maybe the problem is the timer thing I dunno, but I will test it today.

I guess this thread is now useless xD