I got this on the level where you fight the clones of yourself (not sure of the level number, maybe around 40). Bust open a bunch of the clone vats and there will be a ton of enemies on the screen at once. There will also be a lot of shooting going on so there's a good chance that there will be a bunch of powerups at the bottom of the level. Head down and see if you can get a multi-rocket powerup. If there's not one there just die and try again. If you do get it, head to one of the upper corners of the level and don't shoot so you get a bunch of bad guys following you then turn around and blast them. You'll get a bigger spread on the rockets if you aim longer. Again, if you don't get it then die and restart. There's no penalty for dying.