This mode is considered the easiest of all three and I strongly agree. I didn't even fail once - at a total of 5 playthroughs.
If you're having trouble, these tips might help:
1. Always run over those boost surfaces, even if you'll run into something afterwards, it's still worth it.
2. Don't bother using the A button, I didn't notice an advantage while using it, in fact it appears to slow you down even more than just scratching a wall a bit.
3. When you've been outrunned by a white enemy, move on the white stripe they leave behind, this will give you a major speed boost, just like a slipstream.
4. When you see green enemies directly behind you, always use the Y button.
5. All other enemies can be taken care of with a barrier, which can be spawned by pressing the B button.
6. The enemies will catch up to you and you can't do anything about it. Use that to your advantage - wait for an enclosed area and take care of all of them at once and use your B or Y button.
7. Don't use up too much of your speed boost powerups, you might be outrunned a few seconds before reaching the exit.
8. When you're using the A button, you will notice a wind-like wall which comes near you from the behind - don't let it touch you as it will instantly kill you.