What resistor should I buy (More than 2kW peak power)
Calculate the average power. A simple average is used for power, not RMS. It looks like your average power and resistance requirement will make a wire wound vitreous enameled resistor a likely choice. The Ohmite Manual of Engineering Information indicates that for individual pulses in the millisecond range, calculations need to be performed based on the assumption that all of the heat goes to raise the temperature of the wire. Apparently heat is transferred to the resistor core and the air more slowly. I have seen pulses of power cause the resistance wire to break without the outer surface temperature of the resistor rising much above room temperature.

http://www.ohmite.com/techdata/engineering-info.pdf