I didn't really find this game very fun, so my goal was just to get the badges.
As I didn't enjoy it I naturally wanted to get the badges as quickly as possible. For this purpose having a class 4 tank when you can use Mega cannons is useful. Before that I suggest sticking to a class 1 tank.
Design the tank so that a small bubble is placed somewhere in the class 4 region (allow you to place Mega cannons)
Place all the mega cannons you can on that bubble. The smaller the tank the easier it is to dodge. Mind you that it will be slow and that you won't be able to dodge properly like in a class 1 tank. (Note: Enemy tanks will aim non-homing shots at the center of your tank even if there are no bubbles there)
The good part is that a mega cannon has a fairly high rate of fire, deals 4 damage and costs 1 gun point. It doesn't take that many 40 damage shots to take down the big tanks.
I used less than half the time when I skipped from fast class 1 tank to this design.
 "Bubble Tanks Arenas"){kind=icon}