A great game.
Wertliq's strat works every time (may just take more time has the levels become more challenging) and once you get a good defense set up, money isn't really an issue. Just keep sending and use the "repair all" function.
Level 3 is probably the most difficult due to your weapon choices. Once you get the sniper and shotgun, alternate with big zombies versus a group of weaker ones. All the badges will easily be accomplished if you complete the game.
After many attempts, here is how I beat the final boss:
Two of his attacks (cross shot with rain drops and lasers) can be totally avoided by staying at either corner of the boss while your ally is attacking. Be careful to NOT get too close or else you will quickly lose all your health.
With the rockets, I suggest waiting on the bottom of either corner and slightly move up. This will make the rockets move up as well, giving you enough room to slide under them as they diminish.
With his constant stream of cross shooting, I suggest staying on the bottom right corner and stay within his pockets. This part especially is a lot easier with the mouse. The keyboard isn't as sensitive.
the best strategy would be to attack while using the tombstones to upgrade quicker. when he goes up in the air to shoot down the bread, i usually use the magic and add more combos on top of it.
It's actually not that difficult. If you're having problems maintaining your tank's level, I suggest leaving the bubble immediately. I give it a 3/5 when you compare it to the 2nd version.
 "Elite Squad 2"){kind=icon}
 "The Last Door - Chapter 1: The Letter"){kind=icon}
 "Bullet Audyssey"){kind=icon}
 "Hacker vs Hacker"){kind=icon}
 "Zomgies 2"){kind=icon}
 "Reactance"){kind=icon}
 "Castle Crashing \"The Beard\""){kind=icon}
 "Bubble Tanks"){kind=icon}