Ninjimp is a bounty zombie in Plants vs. Zombies: Battle for Neighborville and can be found in Mount Steep, more specifically in the Steep Mines. He resembles an Imp wearing the Ninjimp Dynasty costume.
He must be defeated three times to complete the Bounty Hunt. When he first spawns, he will summon two copies of himself. After defeating him once, he will spawn four copies. After defeating him twice, he will spawn six copies of himself. These copies can not be destroyed or else the bounty hunt will be lost.
Defeating him unlocks the Ninjimp Knock-Offs medal in Mount Steep.
Ninjimp's name is a portmanteau of ninja and imp, referring to what he is, an imp ninja.
Air Ron offers a very good hint: the true Ninjimp is the only three-star Ninjimp. Every one of the clones has stars above their heads, while the true Ninjimp is the only one to have three. Knowing this will prevent you from having to guess and will make the hunt simple.
Avoid playing as characters with splash damage, like the Peashooter, at any cost. You don't want to accidentally destroy the clones and lose the bounty hunt. Due to her precision based weapon and long range, Cactus is the most recommended class to use for attempting this Bounty Hunt.