Real Steel

Your first fight in the underworld is against Scrapbot. You simply need to defeat him to earn the achievement. Don't worry so much if you lose initially (I lost first time around) because you earn money even when losing and that money will help you buy new parts for your robots to increase his powers and will therefore increase your chances of beating Scrapbot.

Once you get used to the game's fundamentals you should beat Scrapbot eventually. Just time your punches, guard when neccessary, use your tech moves (+, , , ) at the right time and over time Scrapbot will be a breeze, especially when buying new parts and improving your robot.