Ow wow you've got some plans!
Ok... Mmh....
I'm gonna make a Ruling here that will also apply too you guys regarding Illusions. I do support Illusion use but I'll have to clarrify some things now.
First, if you were to recast the Same Illusion over and over again they will become Immune to it after some time (depending on their Intelligence)
Also, once they interact with it they will find out its fake. For example if they shoot arrows at your Boulder and they pass through instead of bounce off they will realize something is up. And ofcourse, leaving any survivers might earn you a certain reoutation as a illusionist.
Reading Silent Image it states that 'Physical Interaction' reveals it to be a illusion. And once a creature sees the Illusion for what it is, it can see through it.
So to illustrate this might be something that could happen
Round 1;
You cast Silent Image
Orc Archers lose a turn
Orc Leader inspects Illusion and sees its fake, yells that It's fake to the Archers
Round 2;
Orc Archers shoot with penalties (they are basicly firing blind)
Arrows pass straight through said Boulder.
From this they realise its fake.
Round 3;
They can see through the Boulder.
This is Just a example ofcourse and if Fireballs, Arrows & God knows what starts flying out of said Boulder they will put 2&2 together fast enough.
Hope i have been clear enough and cant wait to see what you will come up with