If you're looking for a simple concept...
It should be feasible to use a mosfet to chop the 430V into rapid pulses of maybe 20% duty cycle. The mosfet will be operated as a switch hence it will dissipate minimal watts.
Then you feed the pulses to a smoothing capacitor which powers your load.
By 70W I assume you mean at 230 V. That makes 304 mA, with your load 756 ohm.
There are mosfets and capacitors made to withstand the voltage.
You'll need to adjust the duty cycle carefully.
No doubt you'll get a lot of ripple. If your load won't tolerate it then the concept will take a little more work. It might benefit from having two-stage smoothing, etc.