Loading question when dealing with 50 ohm impedance matching
You are correct in your power calculation. 


  Wouldn't a usual 50ohm resistor burn out in this case?


Not if it's a 2-watt or better resistor.


  Moreover, how can one achieve impedance matching in this case?


By using a 50-ohm resistor you have achieved impedance matching. If the power rating of the resistor is too low, the resistance will change as the resistor heats up, and matching will fail. As in the first question, the answer is to use a resistor with a 2 watt or better power dissipation rating.