What is the easiest way to get a 50x DC-DC voltage increase?
There is no way (at least on my knowledge) to do that using only DC voltages all the way.

At some point you have to convert them to AC in order to use (for example) a voltage doubler.

In fact, that would be my solution if I have to do this: dc source -> convert to AC --> a series of voltage doublers or multipliers -> rectify/filter the output --> enjoy your HV 

You can look for voltage doubler, it is a really simple circuit, the  only thing tha you  have to be carefull about is the insulation: it havs to be good, really good

UPDATE : you can look for voltage doublers or voltage multipliers, here is a good explanation:
https://www.electronics-tutorials.ws/blog/voltage-multiplier-circuit.html