Yeti's second mission is incredibly straightforward. Get your feet wet to activate it.
Your objective is a rooftop just far away enough to be worth driving to.
Jump up to find the LEGION power unit, and kill the helltroopers on guard.
Be sure to keep up a mix of regular, special, and melee attacks. As the mission and dialogue wear on, Mixer's meddling will power-up and depower Yeti's various moves, so the variety will help keep you lethal.
Just keep running around the roof, killing off wave after wave of enemies as they appear appear. You'll face all the trooper types, including a bufftrooper and a shocktrooper before the commander shows up in the final wave.
Kill him and any stragglers, then destroy the machine itself to complete the mission.