How do I drive a MOSFET working under high voltage while keeping it insulated from the rest of the circuit?
Further down I'm suggesting something that should work in a lot of applications but maybe not yours (due the the high gate capacitance of the MOSFET). Because of that I'd consider using a DC-DC isolation module to provide isolated power to gate circuits at the MOSFET. Here's a circuit that is 90% indicative: -



B2 would be supplied by the DC-DC converter and U1 can be isolated as well although in this particular diagram they show both sides of the coupler grounded (directly below U1). DC converter like this would be OK: -



It can continuously supply 250mA and this can be used in conjunction with a reasonably sized electrolytic capacitor to provide the high current surge needed to charge the gate.

I'd also consider the simplicity of using a photovoltaic opto isolator such as the Vishay VOM1271. 



It can switch on in 53 us into a 200pF load and produce a drive voltage of about 8V making it suitable for a lot of MOSFETs. Of course if the MOSFET gate capacitance is 2nF then it will take about 0.5 milli seconds to turn on.