Geary can be a real toughie. She is the 4th boss. Her race is on Android Alley of Planet Tekknee. While racing her, two small streetsweepers also race, and help her out. She will leave these homing barrels which can flatten you, while the streetsweepers leave puddles of soap which make your kart hard to control temporaily. All 3 of these guys are pretty fast, but their weakness is defintely their vulneribilty to weapons. While they may be fast, their weapons are not shot as consistentely as the other bosses, making them very easy targets. Use combat, not speed, to take them down.
Krunk is the 1st boss and his race is on Jungle Boogie on Planet Terra. He shouldn't be too difficult if you use Crunch or Tiny. This track's turns are fairly wide, so speed is your top priority here. He'll always boost ahead of you, no matter how good your acceleration is. He has one attack and one attack only. He'll leave these strange plants behind him. If you hit one, it'll latch onto your head and slow your car down temporaily. To get these off, hop constantly. The thing about Krunk is, once you're ahead, you stay ahead. He seems to be quite slow and unaggressive. This should be an easy win.
Nash is the 2nd boss and is raced on Deep Sea Driving of Planet Barin. He's a tad bit tougher than Krunk was, but it seems as though it's more of the track being a problem than Nash himself. He'll go a lot faster than Krunk, so it's going to take more than pure speed to pass this guy. He has two attacks. One is leaving these spike traps behind him. When hit, you spin out. He'll also let out chattering metal jaws. They spin you out too, but they're harder to dodge as they'll actually move. If you manage to hit him with a few weapons and use the shortcut, you should be able to pass him with ease. A wrong move, and he can pass you in no time. Be aware this track is littered with obstacles.
This boss race is very unique. Instead of only one, you're racing two bosses. These guys are the 3rd boss. They are raced on Out of Time of Planet Fenomina. They both have their individual attack. Little Norm will blast out bubbles with surround you and lift you up into the air, slowing you down drasitcally. Big Norm lets out these strange squares which slow you down. Both can be demolished by some simple hops though. This is quite an easy boss that requires almost no skill. They seem to be horribily slow and passed easily.
 Velo the 27th
This is by far the hardest boss. He proves to you he is the best racer in the galaxy. Period. When racing him, two alien cronies will also be racing on small hoverboards. They are the 5th boss, and they are raced on Hyper Spaceway of the center of the galaxy. They will prove quite a challenge. All 3 will drift off far ahead of you at first, but you'll catch up. Don't worry. The aliens will leave normal Static Orbs while Velo launches out infinite bombs and homing missiles. That's right. Homing missiles. Their own Static Orbs will not harm them. They are very consistent in their attacks. The aliens will not be easy targets. However, Velo is an easy target due to his weapons not being able to block. Hit him with a missile or bomb and you'll drift ahead of him. While ahead, make sure you stay ahead by going as fast as possible while storing a good powerup. Otherwise, he'll merely speed past you and leave you in the dust. The aliens whill prove to be a tougher challenge to hit. However, they're slower than Velo, so speed can outrun them. Be careful.