If the output voltage should be 0.3V then you can use the available +3.3V supply.
Connect one leg of the series resistor to +3.3V output, its other leg goes to the anode of the Schottky diode and the cathode goes to the negative point of your +3.3V supply, ok? Simply you use the Schottky diode as a Zener diode.
Small and medium power Schottky diodes have a forward voltage drop of 0.15-0.4V, depending on how much forward current you pass through.
For instance, see the Characteristics Curves of this Schottky diode here: http://www.semicon.toshiba.co.jp/eng/product/diode/selection/topics/1176538_2381.html
and you see 0.3V forward voltage drop around 35mA forward current, ok?
Now you can calculate the resistor value like you calculate for a normal Zener case, considering your output current need around the 0.3V output and this output is from the diode two legs of course.
Did you think this solution with the Schottky diode?