Main Storyline do case missions can't miss it.
SPOILERS AHEAD!
*** Spoiler - click to reveal ***You will be on the case: THE FACTS
You will be fighting Stacey or Agent S rather.
Make sure to bring plenty of food and weapons as you will probably lose a bit of both if it's your first time.
The fight isn't tough it's just long and it stretches into 3 parts.
First part: You will need to knock out the robots arms. Avoid the arms and the incoming rockets as you wait for the arms to hit the ground and stay there. Run behind the arms and attack the back sparking portion. When the bar goes down, you'll go into the second part of the fight.
Second part: The arms will break off into robotic pincers that slam into the ground. When they get stuck in the ground, run up and attack them. Once again, avoid the rockets that she'll shoot out as you attack the pincer. When the bar goes down, the fight goes into the third and final part.
Final part: You will unlock acetylene tanks on both sides of the robot. When the head lowers down, jump on top of a box and push B to unleash the rockets. Immediately jump down (the bombs won't hurt you) and run toward the head as it hits the ground. Jump inside of it and hit B to initiate the fight with Agent S. Prepare for a button mash as you will need to hit 5-6 buttons to hurt her. When her health goes down to nothing, you'll have defeated her and have gotten the achievement.