Rap punchlines are to hip hop like slam dunks are to basketball: if done correctly, they bring a crowd to their feet. However, if you mess up, it's just about the most awkward and embarrassing situation imaginable.
Freestyle battle rap punchlines are given a little more slack than their written counterparts, since they are off the top of the head. A decent freestyle rap punchline can bring the house down, and if you screw up, well, what the heck, you were just freestyling anyways …
Let's take a look at different types of rap punchlines:
The set up / smack down approach. In keeping with the basketball analogy, think of a solid punchline like an alley oop: the first line sets up, the second line slams it home. For example, "I respond back, just like a restaurateur / leave your ass feeling served while you're eating your words."
Internal rhymes . When you can string several multi-syllable rhymes together, it makes the phrase that much more deadly. For example, here's a line TheSaurus, one of the greatest battle MC's today, said: "I'll smoke you like a purple eighth, and give you an early date at the pearly gates." Ouch …
The reversal. This is where you take your opponents line and use it against him. For example, let's say someone calls you a nerd in a battle. You can respond "This guy claims I'm a nerd, well I guess that's true / A nerd with words and verses much smarter than you!" OR "You call me a nerd 'cause of big words in my rapping? Son, your IQ's like your skill … totally lacking."
I hope some of these techniques have given you some ideas on how to construct solid rap punchlines. Keep them in mind next time you battle; you never know what may come in handy.