Just put 1 small bubble in the middle and stack it with Piercing Cannons and later Exploding Piercing Cannons. Just aim, a spawner dies after 3 hits and if you just keep moving around you don't get hit.
4/5 : Great game,complex in a good way but a lot of minor problems that can take the experience away : The Colours : Please give us a way to change the colours,my eyes are bleeding. Multiplayer : When I saw this I assumed it would be multiplayer,sadly this is not the case. Tank Building : I see no reason for different bubble sizes since you can just use the Huge Base Bubble, Instead just make 1 Bubble type and decide the class based on how much of the circle is covered rather then that system. And try to make the Symettric system a little more tolarant. I hope you'll look into these for the sequel. It could make this game a lot better.
You can click on the Main Method,Function 1 or Function 2 windows to select them, then you can just click on commands and they'll appear in the selected window.
First of all,great game,in my opinion the best tower defense game I've ever played. There's not a huge amount of towers,no maze needs to be created, a balanced skill tree,nice graphics,the different ground system,... They all make for a very earned 5/5.
Secondly there are some things I would add,though much of these have been mentioned : Hotkeys (Mostly for the spells),Reset Skill button (There are some levels where your normal balance doesn't work),Level Editor (Should be very easy with this game), A survival level and a special bonus for maxed towers.
I hope to see more of this. ;)
Hint for lvl 4 : build a zooka as soon as possible and put alot of infantry in top or bottom lane,then the zooka should blast their tank and claim victory
 "Flight"){kind=icon}
 "DJManiax"){kind=icon}
 "Man in Gap"){kind=icon}
 "Heir"){kind=icon}
 "Bubble Tanks Arenas"){kind=icon}
 "LARRY and the GNOMES"){kind=icon}
 "Lightbot 2.0"){kind=icon}
 "Deep"){kind=icon}
 "Onslaught2"){kind=icon}
 "Ultimate Crab Battle"){kind=icon}
 "Caesary"){kind=icon}
 "Async Racing"){kind=icon}
 "Robot Wants Puppy"){kind=icon}
 "Zening"){kind=icon}
 "Cursed Treasure: Don't Touch My Gems!"){kind=icon}
 "The Idle RPG"){kind=icon}
 "The Next Floor"){kind=icon}
 "Warfare 1944"){kind=icon}