My interpretation after reading pg. 72 of the rulebook.... let me start with this... in no other circumstances in the rules do shaken, stunned, or immobilizations effects stack. You can with wpn destroyed results eventually get a immo out of it <-(rare. pg67). So if that is the case no matter how many times you stun him he only loses 1 attack...so it is A+d3-1 = still dead whatever he hits.
The "to minimum of one" part wont really come into play with SM Dreads, but will more affect Sentinals, War Walkers, etc.
Just how I would play it