Wow really great guide. there is one thing that i would like to add but it's just a way that i found to level up your tanks well. I found that when i first started playing i wouldn't care the direction i went, and just go in a direction and stick with it. After a while, i discovered a much much easier way. As most people probably know, the enemies get harder the farther you get from the center. So a very easy way to level is to form a square around the center, and just add a layer at a time. I don't know if this is confusing so i think i'll add a diagram.
you start like this:
O
next step should be this:
O-x
then here:
O-x
x
and here:
O-x
x-x
you continue in a manner relating to this:
O-x
x-x-x
and just continue sticking as close to the center as the map allows. eventually you'll be getting to something looking like this:
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-O-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
x-x-x-x-x-x-x-x-x
This appears to be the easiest way i have discovered, and as an avid player of bubble tanks 2 (i've filled my bubble-o-pedia) i have put it to good use.
Hope this helps you all!