(works best with the last 3 tanks you get)
1) Find a bubble with 3 unexplored bubbles next to it (the explored one will be the one you used to get to your current bubble). The bubble should have some fairly powerful enemies
2) Let them kill you
3) You will be taken to a bubble next to the one you died in. If you're at a high enough level, it will be full of life carriers, bubble balls, and various small enemies. Kill them all, then go to the other 2 bubbles next to the one you died in, and kill everything in those 2.
Unless something stupid happens, you should have a lot more bubbles than you started out with. For example, I had a fighter #3 tank, and I got up to fighter #4 and half way to the next tank doing this.