Pressing start on a second controller activates Plankton - using the second controller, target happiness nuggets (the gold coins) and press A to turn them sad and they will become your ammo.
Target the enemy (again using controller 2) and press RT to blast them - the amount of ammo needed to defeat each enemy varies, but it's usually three to four blasts.
You don't need 2 players to do this, if you stand above a group of enemies you can pick them off one by one, and they can't reach you. Just make sure there's enough ammo about - enemies sometimes give you ammo when you die, or you can also 'shake' them using the left stick - left/right a couple of times then up and down to slam them to the ground and be awarded ammo.
Just rinse and repeat until you have 100!