You might want to redefine the rules of the game and start over. You aren't to DDoS it, but you need to take the server down. Why take it down? Why not just change the wallpaper remotely, once you have cracked the system? It doesn't *really* matter, but I'm using this to help you in other channels...think beyond the boundaries, the bad guys certainly will. That's why you've gotten the suggestions you have.

To be honest, with the restrictions you two have placed on this game and in this setting your friend may be right and you may be wrong. You are dealing in too many finite points, and Security is not a finite (science|school of thought|art).

Why can't you DDoS it? The end result is the same. If the point is to exploit the box ONLY via RPC vulnerabilities, and it is up-to-date, then you may have already lost...UNTIL the next RPC exploit is found.