Without going into too much detail about the casting of illusions and changing the system; I wouldn't change the way that the character is using that spell. The system allows for this to happen but it also allows for you as a GM to react to this way of playing. The world would react to something like this by for example:
1. Somebody who is this powerful? Send an assassin to cut her down.
2. The local villagers would become afraid of her and flee. Leaving her in a castle without food or servants etc.
3. A powerful caster is recruited. He goes along with the attacking force and casts invisivble on himself. The moment the giant appears…. cancel essence.
The problem shouldn't be in the way the players uses the rules but in the way the world reacts. I have a player who is Always looking for loopholes. He finds them and uses them once or twice. Then the world gets wise and reacts.
