You mean the level 14 "rocket race" where you fight enemies that have ~2x your survivability and you have two wonderful feeder allies who go 2-17 and cause you to lose even if you are 22-11?
How to defeat the grinding/money grabbing freemium game: Open two autoclickers, set one to click near the bottom in the middle where the "GO!" button to start a mission is. Set the other to click a bit to the left where the "replay mission" button is. Select a mission with decent income that you won't fail, then leave the game on overnight and you should have enough in game currency to either finish the game or get substantially further, after which you can repeat the process on a later mission.
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 "Bloons Super Monkey 2"){kind=icon}