As i can see, you don't have much knowledge of triggers because a basic triggerer should know that so i'm going to tell you part to part.
First you add the action that Pharaoh_ gave you, and when you want to change the unit visible again, you must change the Transparency to 0. For the Evasion spell for Mortred in DotA, they use that Action.
This may be an unnecessary post but i just want you to learn. Also, you must have like lessons to be better in triggering. Like if people want to help you, they should not give you the whole thing 100% done, you must do some yourself.