Kill the Metroids while artificially morphed without any movement items. Dodge the Rinkas then place a Power Bomb on the lower middle section. Place a second in the right corner and use it to boost up to the right. Without moving too far to the right, place a third Power Bomb and then move back to the left pit after a short delay. Place a fourth in the bottom right corner again and it should explode just before the Metroids get to Samus. Occasionally, the right-most Metroid gets stuck and Samus needs to go to the right and use two more Power Bombs. Note that it is necessary to exit G-Mode before exiting the room in order for the Metroids to remain killed.