Set last created unit to a unit variable. Like, DummyMirrorCaster = Last created unit. Issue an order with no target for DummyUnitCaster to Orc-Mirror Image. Now, I have read several times on this forum that ordering an ability like this will always use your custom ability instead of the regular one, but I've tried ordering custom abilities and it doesn't work. You need to specify the order string so it distinguishes your custom mirror image from the regular one. The reason you'll need the custom one is so you can have a lot more copies made in order to surround someone. Ask others how to do the order string business.
Note: If your map won't be using the regular mirror image, then you don't need to worry about changing it, so you wouldn't need a dummy mirror image ability--just change the regular one to 6 or 8 copies and remove their extra damage taken.
One last thing, you'll need to pause the target unit while the images are being made if you want this to work when the target is running away. Unpause it after the images are made.