The railgun is found after clearing the north world. Once fired, it penetrates enemies. Use the same tactic as the five bomb kill achievement (note you don't have to kill the enemies for this one). Wait for a room with a handful of enemies then dash around until you have them huddled together. Retreat back then take aim as they run at you. It may take a few tries but again, should come naturally if you use the railgun enough.
If you're having trouble, the southern world has plenty of rooms with numerous enemies to get through.