This guide will help you find each car's weaknesses in Need For Madness.
Tornado Shark: Wasting this one is quite easy. He has low powersave, so hit him and don't let him get away. He can be quite durable though.
Formula 7: This is pretty straight-foward because he is very weak. Catch him on the rare occasion of having low power, and hit him from anywhere. If he is driving towards you, try colliding him head on. This will send him flying, and he will crash into a wall. Sometimes he will be wasted upon the impact with the wall.
Wow Caninaro: He is strong, but doesn't have the best control. Catch up and whack him from the sides, or better, the back. Even more preferably, at an angle (back-side), which makes him do a few rounds in the sand.
La Vita Crab: She is relatively weak, so just blow her from any side repeatedly and much of the time she will try and go for you, only to be wasted. If she flees, chase her down.
Nimi: Wasting this one is very easy, as she has no more resistance than a tin can, also she is very slow. Whack her about 2-4 times from any side and the job is done. Any car is possible.
MAX Revenge: Try hitting him from the front or back. It will send him flying. If you are low on power, hit him from the sides, or chase him down as he doesn't have the best power saving skills.
Lead Oxide: He is tricky to waste, due to his strength. Try hitting him from the side as he is a "long" car, and his structure and handling causes him to spin around quite alot, though he gets lots of power from it. Don't hit him head on, as the front will absorb all damage.
Kool Kat: Although she is quite durable, she is relatively easy to waste. Always hit her from the back, as her sides are resistant to any collision. Keep whacking and chasing her, as the low acceleration is her weakness.
Drifter X: Try driving right into him, then making a sharp turn using spacebar while turning. Due to his terrible handling, he also tends to do donuts. Smack him continuously, until he goes ablaze.
Sword of Justice: The main strategy is to have full power, and hit him from the side. Never hit him from the front because his ultra-convenient spikes will give you a dent. However, larger cars can hit him from anywhere, as their wheels/scoops ride straight over the spikes and barricades, and are free to wham right into him.
High Rider: He is quite durable, but will be easy enough to waste as long as you know what to do. Hit him from the side or back, and he spin him out of control for a while. Don't attempt a head on collision, as even larger cars will come out with a bruise, due to his structure.
EL KING: Always hit him from the side. Hit him there continuously, then finish him off from the back. Don't try to hit him from the front unless you have full power and a lot of momentum, or you might come out looking like a flying toaster.
Mighty Eight: Even though she is pretty fast, she is relatively weak. Hit her from the front side then smash her against a wall or a car, and afterwards there won't be enough to fill a bag. Also, her AI, which oddly has suicidal tendencies, will help you waste her.
M A S H E E N: Despite his superior strength, wasting M A S H E E N can be relatively easy, due to his slow speed, if you know what to do. Be careful not to be scooped up, though. Hit him from the side or nudge him from the back, as his workings are quite delicate, but NEVER hit him from the front. The only exception is DR Monstaa, who just goes straight through M A S H E E N, which is useless.
Radical One: This one is tricky, as he/she can send you flying. Hit him/her from the back or the sides, and make him/her spin. If he/she is going slow, hit him/her from the front when you have full power. He/She may rebound you and hit you to the other side of the field. Many larger cars often end up sliding over him/her while doing a head on collision.
DR Monstaa: DR Monstaa is really tough to hit. If you want to waste him, hit him from the back, which is his weak spot, or from the sides, but don't go under him, that's dangerous. Also, avoid any head on collision with him. If you do, there won't be much left of you.
One thing you can do is that you should always have a good power supply. If you ram into another car with low power, chances are, you won't deal a lot of damage, or may be wasted instead.
TIP: Almost all cars take more damage from the sides than hitting it from any non-weak spot angle.