To unlock the Come Get Some Achievement, just Complete the SP campaign on Hard Difficulty. If this is the first time you've played through the campaign, you will also unlock the achievements for completing it on Easy and Normal (Piece of Cake and Let's Rock Achievements, respectively). Beat the campaign on Insane Difficulty to unlock the Damn, I'm Good Achievement. For more info on Duke Nukem Forever Achievements, check out this post. For help with certain chapters, see these Duke Nukem Forever Walkthroughs.