The Evasion and accuracy.

How would I make it where the higher your agility on your hero the more evasion and higher the dexterity the more your accurate instead of missing attacks from another units evasion how would I make this? or the dexterity how would i counter evasion with dexterity?

First of all, this is not Diablo.
You need many levels of evasion spell booked and dis-iconed, then you need a formula for your "Dexterity" possibly exponential decay.

How about faking evasion by Unit takes damage and healing?

Because if they're on 10HP, then it might not heal them before they die? Or am I wrong about that?

They would die, but it would be the easiest way.

Also you would need to implement a real attack detection system for that such as Vex's so that you don't block spell damage. Unless you want spell damage to be evadable..

Actually, the damage event happens before the life is actually substracted.

You could also dynamically cast short-duration curse spells on unit is attacked event.

Or, if you have got no dispel abilities, cast the appropriate curse ability on each unit, and then just have evasion and curse.