This has to be done after you beat the game and unlick Challenge Mode. This solution works if you have found enough gold bolts to unlock infinite ammo.
Complete the story until you reach Aridia, talk to Skid and turn on infinite ammo. Go to the left of Skid and you will find two (2) sand shark spawners. Equip a gun like the pixelizer with a short range and let the sand sharks come to. Keep killing them without getting hit until you reach a multiplier of 20X and the trophy will unlock.