So the round reward is always 150 credits, you don't get reward for round 1 or 10, you start with 600 credits, and the reward per minion is 3*waveLvl. Number of minions in a wave = 5*(waveLvl-1) + 15. So maximum gold you can get without credits generator is 9225, way more than enough to win, you just have to spend it efficiently. Tactic I found that will always win is to start with freeze/catapult combo (just try to cover as long of a line as you can), save up enough for a second catapult, then save up enough for an electro tower. After that, you can go straight for an ultimate tower, then buy a second later. Have fun!