2 : Deactivating its weapons is not enough - it doesn't prevent the electrical blast. Whenever one of the mages behind him ( or yourself ! ) use an electrical attack on it, it gets charged. You have to use the "discharge electricity" command which should be angry angry sleepy, or activate the 3 levers starting from the one closest to it.
Better yet : use the Vent the Steam command ( angry happy angry I think, not sure ) , which will make a cloud of smoke appear around it. Whenever one of the mages overcharge the robot with electricity, it will get stunned ( it will still be charged so you will still need to discharge it in order to avoid painful death whenever it regains control ).
To be honest I'm not sure what exactly the weapon deactivation command actually deactivates since the Arhumaster still attacks and kills people.