Try this, you Warcraft fans!
My personal best score was 238K.

Enjoy

Level 40 is one seriously tough mo-fo. I had like 200 lv4 fire turrets and I still had to buy lives.

You said it Sing, that thing gets rolling and hard to get at on 40, laff

Laff I can't even come close to getting to lvl 40

It's difficult.
There's a couple of ways to do it.
One that will get you a high score and one that won't.

ya, it's ridiculously tough

If you buy rockets, you can do it.
But then your score is going to be ~8000.
If you do it with cannons...
238K.

You don't need rockets, you can go for 80k and use fire turrets if you want. Basically you invest in interest until level 35, then buy your element right before the final levels. Sell your cannons, buy lv2 and lv3 fire turrets and you're golden. You don't even need very many, you'll end up with more than enough cash to do it if you're careful.

Beating it w/ the 200k+ requires you to carefully manage your cannons, always invest in interest and stack the heck out of everything. I've done it once, I didn't write it down... but there are tutorials out there that can help. All you get doing that are the basic cannons (pain in the butt). I just got 140k on my last run, it required like 300+ cannons double stacked (which the author considers a bug) and 2 passes thru the final level (buy a lot of lives) to pull off.

Needless to say it isn't easy.

It never is. Took me forever to micromanage between rounds.