I've found 6 chaos emeralds (I can't get the last one!!), but I can't beat..

My advice is to always be running back and forth across the platform while eggman nega is in front of you. That way, it's easier to stay ahead of him and dodge with the super speed(down plus B)and it allows you to quickly turn around and attack his hand while it pauses for a moment on the platform. Unless he uses those weird purple electric cages, you just have to do your best to avoid touching where the electricity hits the ground by tapping the arrow buttons in the direction that the cage is moving. Also, when he uses his lazer eye beam, it's not the light itself that you avoid, it's the little explosion following the light where it touches the ground. Just jump over it when it comes near you. When he moves to the side, jump over the shockwaves from his arm(they will get harder to avoid the less hit points he has) and attack when it stops hitting the platform. Now the only way to actually deal out damage to his hit points in to attack the robots head by running up it's arm. In order to avoid getting thrown off his arm, TAKE YOUR TIME You get a warning just before Nega is about to throw you off. IMEDIATLY stop moving forwards. Stay on the arm and just hold the down button. As his health decreases you might have to stop twice on your way to his head. Once you get him down to one hit point, he starts charging into the platform. Unless you attack his head while he is stopped DIRECTLY in front of the platform he will continue to charge and eventually (although I'm not exactly sure, I always got him it two or three tries) destroy the platform completly. It's really just a matter of hitting him in the right spot. If you target his head every time he charges, you'll get it. As for coins, you will always lose a few in the end. Don't bother trying to save uncollected coins by only grabbing them when you need them. I find it's easier to hold onto them all. If you do lose some, do everythng you can to catch them. Even if you only hold onto one, which can actually be easier sometimes. If you still have some trouble, just remember to slow down and take it at your own pace. You only need to worry about time and speed when he's down to the last hitpoint. Good Luck!