Due to the fact that illusions die immediately after the target enemy dies, it is very hard to even shatter anymore. 90% of the time I try to build up 3 illusions, the mob dies. And when fighting multiple targets, even worse, cause now I have things still attacking me, i have 0 illusions, and I wasted all the skills on the previous target... so I'm screwed.
I believe if that you really want to keep this requirement, you should also let the illusions stay while you are still in combat instead of destroying them immediately when the target dies. Sounds like a fair trade-off to me.