i mean he's small enough to sneak past
if the average bikini bottomite is about the size of an adult human hand, that leaves plankton at about the size of a pencil eraser
plus he could probably build a water suit to live on land
with his size and said suit, he could very easily sneak behind enemy lines
plus with machines such as the robot from the movie, he could use technology far more advanced than chemical warfare, tank shells and bullets