My epic tip(not really epic but...) to beating lvl 15 easily- Use your rockets to easily destroy the enemy. Just spam a few against any bosses using rocket button twice to explode the rocket quickly. Destroy spawners in 2 hits and bosses in 2/3/4. Use other weapons against the weaklings. Figured this out after i got super cornered by 3 bosses and spammed half of my rockets to kill them. I won on my first try with this. Be sure to max out the weapon #5, bring full ammo, and some other stuff..
To add on rka's comment, try getting some repulsors to help push back the creepers. Personally, all i did was rush all the way to the top quickly in lvl 10, then i destroyed some decayable blocks and added 2 shields, 2 repulsors, a lot of blasters, a few rockets, and the teleports. if the super drone destroyed anything i would replace it. While i let the volcano from exploding I did some random stuff to destroy the creepers on the right....
Tip: When fighting against large numbers of creepers, use multiple rockets(multiple as in 3-10) to take them down. If you think the creepers in the bottom are taking too long to defeat at lvl 5, put some rockets on to the left and right rooms/sides of the creepers to speed up the process.
go to the creeper world 2 site and look at the fun accolades..one example
"My girl friend told me to choose between her and playing Creeper World. I told her I'd miss her, maybe."-....and another "This game is over 9000!".. these people are interesting, especially the person who said the 1st example..
 "Awesome Tanks"){kind=icon}
 "Shadow Regiment"){kind=icon}
 "Creeper World 2: Academy"){kind=icon}
 "Santa's Penguins"){kind=icon}
 "Snow Tree"){kind=icon}
 "Atomic Strike"){kind=icon}
 "Epic War 5"){kind=icon}