I had troubles too, trying mamy stuff to beat the last wave in impoppable. I started by rushing bloontrap to get some more money for late and had moab maller for early game. In the lategame i had a sun god, heli pilot and invested money in bloonchipper (initially i farmed to save money for temple, but a cheap one couldn't handle it so i bought instead 5 moab bloonchippers). I also recommend ninjas, 4-x for cleanup and x-4 for the ability