Find Nefarious is a mission in Up Your Arsenal taking place in the Leviathan, docked at the Zeldrin Starport. After Ratchet met up with Captain Qwark, the two traveled to the Leviathan in order to find and confront Dr. Nefarious.
When you land in the ship, consider using the Gadgetron vendor to purchase any weapons you have not yet already obtained (if you do not have one of the Rift Inducer, Annihilator, or Disc Blade Gun, they will be very useful here). Ahead of you is a room with several soldier bots patrolling parked tyhrranoid dropships. One good strategy is to just aim the Rift Inducer between the enemies and take many of them out before even approaching, or to just use the Flux Rifle or Annihilator to similarly take them out from a distance. Many other weapons are also viable, such as V3 or V4 versions of previous weapons.
You will eventually come to a bridge, at the end of which are ninja bots, handle them and destroy any soldier bots that appear atop the next platform before heading upwards. Finally, defeat the next group of robots and head to the final elevator at the end, where a cutscene will commence, leading into the next mission, "Escape the Starship".