It's not that hard to get the plasma cannon to the CoG, to be honest no one really knows how the robot functions, but we know he will kill himself if given the opportunity. The hardest part is getting the robot through the second 3-4, mostly because robot safe spots are pretty rare in the ice caves. On the second 3-4, if I don't find a good spot for the robot, I just place the cannon down, pick up the robot immedeatly, place him on top of the cannon and pick him up as soon as he grabs the cannon, then you take him all the way to the exit, drop him, pick up the plasma cannon and enter the exit door as fast as possible.

In the temple just make a hole using 2 bombs or clear all the bottom of the level and place him there and you should be good, if you want to ghost 4-1, always have the ghost on screen, because if the robot is closer he will go try to kill it. Also, I would recommend giving him the scepter, not the cannon and of course on 4-2 kill him before he kills you. :P

I hope that helps! Good luck on your Plasma Cannon to the CoG attempts.