Geez, what a time waster...
It's best to play such games on their "home" server, armorgames.com. The game size is a little bit larger there.
Exploit a bug in game design: Buy tower. Sell tower. Repeat. You get experience for buying towers as well, so you level-up in no time to get the flak cannon + start with more money.
a.) Build mazes with the simple turret (don't upgrade) to slow down enemies.
b.) Use flak cannon (upgrade!) or later missile launcher (upgrade!!) only.
c.) Replace some turrets with the green turret (upgrade!) after the first few waves to slow down enemies. It helps a lot when dealing with tanks or bosses.
d.) Levels with two entrances... use the turrets to direct the penguins to the other entrance.
Too bad the game has some bugs... bosses do not appear (often), airplanes fly off screen and cost lives (lvl 4 und 6), invisible turrets (sometimes). But it's still fun.