I think a big power pneumatic punch would be good but you'd need a way to hold the other guy still to transfer most of the shock. For instance something like a pincer of big nipper or can opener with a big ram either side running full pressure. Get hold of them and then punch from both sides. Or some sort of design like razor or tiberius where they come up onto the robot only to be repeatedly punched by a jigger pick type weapon on the end of the arm. The fact you have hold of them would hopefully rattle them about a bit.