Nappa is Vegeta's right-hand man and partner along with Raditz. This battle is challenging because it's the last battle before the final battle against a partner of Nappa's, Vegeta. You must defeat this evil Saiyan before starting the epic battle against Vegeta, which, depending on your win or loss, will affect the Earth in a good or disastrous way. But of course, as best friends of Vegeta, Raditz and Nappa will stop at nothing to protect him. Because of this, you must get past Nappa to defeat Vegeta and protect the world.
The last battle before the final battle, the battle against Nappa isn't an easy one. Most likely, it'll take a few tries to get it mastered. This is also a good warm-up for the final battle. If you can't defeat Nappa, then you can't defeat Vegeta. Go back to training to buff up your profile, namely Ki and HP. The short training mini-games may not be the funnest, but they can be a big help in the long-run.
Nappa can go really overboard when it comes to blocking. Because of this, you shouldn't waste much of your Ki on flashy moves. Uselessly, it'll be blocked. Also, Nappa has a very HP. Each move may seem like a feeble attempt towards victory, but each move is also a step farther towards the right direction. Just keep at it, and when he loses at least half of his health, the rest will seem much easier. Start with something like a simple Ki blast, and soon enough, you'll be able to execute a Kamehameha that won't be blocked.
Defending can't be too hard when fighting Nappa. Attacking is probably the bigger challenge. Nappa is as wary about wasting his Ki as you, and not much of his Ki will be wasted early on in the battle. However, you should be more careful when your HP goes lower. His attacks will get more fierce as you get weaker. All he wants to do is get you out of the way. Try dodging his attacks for the bulk of the beginning of the battle. Block when his attacks get more serious, since a headbutt is easier to dodge than a full-fledged Spirit Bomb. Unlike the battles you fought before this, you will get the first offense. This might help you in the battle, but defending shouldn't be ignored. Remember to be cautious, and do the best you can. Practice for the next battle, which will be the final one. To the right is a screenshot of Nappa performing one of his long-range attacks. It's best to dodge for long-range attacks.