It would probably be easier to replace the grille. To bash out the dents you might have to take out the existing grille which might cause more damage.
Assuming you get it out cleanly or can get good access without taking it out, place the grille face down on a firm but padded surface and then take a wooden or rubber headed mallet and hit the grille where it sticks up. Make sure that you keep the grille steady so it doesn't bounce. You want to transfer the energy into the grille and specifically into the deformed area. Start with fairly light taps and gradually increase the force until it starts to flatten. Too much and you'll cause a dent in the other direction.
Repeat until you're satisfied.