Warning: This game is heroin.
I played this so much. I love this game. I'm assuming this is the one with the monkeys and they throw darts at balloons? If not, then I'm way off.
Not sure about level 8 but I've beaten many of their maps on every difficulty setting. For me, it was key to spend all the money I had. Each round, you should end with no money. Spend it all, you will get more. Add more towers, upgrade the ones you have. It's pretty epic.
Jeremiah 20:9-But if I say, "I will not mention him or speak any more in his name," his word is in my heart like a fire, a fire shut up in my bones. I am weary of holding it in; indeed, I cannot.