Since the last guide was over four years ago, here is a more updated version. Most of these strategies were pioneered by Squidguardian10.

Any%: Play all the way up to level three while upgrading your minigun. Max it out after finishing level 3. Then, instead of progressing, play level 3 until you have enough cash to buy the rocket. Once you have the rocket, you can use it to spam rockets to take out enemies quickly. Watch either Squid's or my Any% to see how it works. Focus more on tank upgrades, mainly armour. Since this requires you to be close for most of your kills, this means that flamethrowers are much more of a threat, and you will need to focus them as they will set you on fire. Aside from that, most of the levels are easy to beat. If you want to either buy the laser or railgun for the later levels, do that. I use laser because I am more used to it for levels 14 and 15. Hopefully if you follow this and have a deathless run, you should be able to get a sub 10 or at least a sub 11 with a little practice.