The main method of advancement is to buy and build up (using credits earned from prior cleared waves) your avatar and weapons load-outs, but not hold onto them so long that you get stuck on a 4th or 8th wave when you fight the bosses. Below are some general pointers to build your burger avatar, hopefully resulting in success. I'm far from expert but this is better than nothing, which is exactly what this guides section has to date. : )

• Don't forget to build up your vehicle's health, damage, speed, not just the damage, speed, and reload times for your weapons.
  
• Don't keep a bunch of weapons from the same class. Pick one or two based on how the rounds spread or bounce, build those up and sell the rest as you go. Each new set of waves / new planet will give you the chance to get better weapons per class as well, so you may want to sell off the prior ones as you start a new wave, then immediately build them up some.
  
• Don't keep more than two loaded up Avatars. Even that may be too many after the first 3 or 4 waves / planets. You will need the credits to max out the better of the two in terms of health, damage, speed. IOW having two good avatars at 75% capability is nowhere near as good as one at 100%. and each notch up the ladder costs a lot more credits. The fewer rounds required to clear a wave, the more successful you'll be; 80s / 90s style fire button mashing will get you nowhere.
  
• Some of the boss fights will require you to go into wait-and-strike mode so speed becomes key as they chase you around. Don't rely on the speed technology, build up actual vehicle (always on) speed instead. Much more valuable IMO and leaves the tech slot open for other stuff like Aegis. That perk becomes more important as you go because there will be times when you need to "shield up and run" to be successful.

Caveat: the above assumes you are not willing to use the typical "cheat" of taking your ramped up avatar and going back to prior planets and levels and rampaging through a bunch of them to rack up credits in order to buy more stuff. If you want to do that there's really no limit to how far you'll get just a matter of hours played. If you're trying to go through linearly though without going back it gets tricky.