Given that Grevious stated that Khazna had seen the radio waves before taking the weights off, he had only as much time as recieving the signal and triggering the explosion would take - and that is to think of taking the weights off (though Khazna should be able to do that pretty fast), actualy removing the weights, activating his flash skill and travelling a distance enough to be within the shield's proximity. Also, add the time for activating the shields here too, as they cannot be switched on before Khazna passes them. 

Though you might as well forget about that all and let the guy live. ;3