The best way to bring drop 700VDC to 12VDC?
using the flyback converters is not an option. 


Why?


  Most of buck converters have the maximum input lower than 100VDC.


Buck converters work well for input/output ratios up to 10:1 or a bit more, but 700V/12V is 58:1 which is definitely "not the ideal use case" for a buck, in the same way that using a hammer to drive a screw is "not ideal".

What you need is a transformer based converter, either flyback or push-pull, or other high current topology if you need the amps. This will have high efficiency.

Since 375V DC is lower than the peak voltage of three phase 400V AC, you should be able to use an industrial switching converter that takes 400V AC input. Most AC-DC switching converters will work fine with DC input. Check the documentation.


  The only solution I can think of, within my knowledge, is to use a voltage divider


No... Try to calculate the power loss in the resistors, it will be obvious why this is a bad idea.