Yes, it is quite normal to be terrified when a 700kg load swings wildly under you.
But why did it begin to swing ?
If it is due to aerodynamics, then slow down. If that doesn't work sufficiently, land and repack - heavy part of load will try to point in direction of flight (draggy part trails). If that still doesn't work you will have to get a net to carry the load in. The net will act as a spoiler and kill any tendency for the load to fly on its own.
If it is not due to aerodynamics, then treat the load as a pendulum. Take the energy out of the load by lowering the collective and/or follow the load by banking to the same side as the load swings. The last method can be modified into an ever so slight application of pedal to same side as the load swings, i.e. starting a turn "into" the load.
You will eventually develop flying skills to unconsciously cancel out a non-aerodynamic swing. Loads that want to fly somewhere else will be with you forever.
Ray Poutry [sp?] wrote a number of articles about sling technique in Rotor & Wing many many moons ago - I think they were published in a book.