Hi, i'm on the 5 seconds to die mission, and theres a silver key that I can't access because theres an unkillable robot in the way that hurls rockets down the tight corridor like theres no tommorow (i've used 4 double rockets, and half a staffsworth of the staffs lightning attack, and it still doesn't go down) is there some trick that i'm missing?

If you are talking about a robot guard (default robot), you can kill it with a few missle launches or the hand of god.
Otherwise you are facing a Ballisikraft, those are unbeatable. Take the key when the bot is out of sight.

Indeed, the BallistiKraft are unkillable. Also, you'll meet a few of them, but not too many.
As I recall, it was stated in Dark War's manual that those can't be destroyed, so I don't understand why it's not mentioned in the new Rise of the Triad's manual.

Figured it out a few months ago, but I thought i'd leave the solution here.

IT appears that the robots were bugged and weren't moving along their patrol path, thus being impossible to bypass. The solution is to restart the entire level.