Use the first map imo, it's easiest. Use mostly spike shooters (fully upgraded), with one monkey toward the end to pick up strays. By lvl 20 have a cannon to break up the metal bloons which spike shooters and apes are completely useless against. By lvl 36 have a monkey beacon (radar thing) and upgrade it to allow for monkey storms. Also have 1080 money available to call a monkey storm after the MOAB gets broken apart. Note that it will be wasted if you haven't broken the thing apart first because even the monkey storm won't clear an MOAB. From here on it's smooth sailing. Keep at least enough money budgeted to run a monkey storm or two and spend the extra money or more fully upgraded spike shooters. I beat lvl 50 with over 14000 money to spare.
Use the first map imo, it's easiest. Use mostly spike shooters (fully upgraded), with one monkey toward the end to pick up strays. By lvl 36 have a monkey beacon (radar thing) and upgrade it to allow for monkey storms. Also have 1080 money available to call a monkey storm after the MOAB gets broken apart. Note that it will be wasted if you haven't broken the thing apart first because even the monkey storm won't clear an MOAB. From here on it's smooth sailing. Keep at least enough money budgeted to run a monkey storm or two and spend the extra money or more fully upgraded spike shooters. I beat lvl 50 with over 14000 money to spare.
 "Idle Mine"){kind=icon}
 "Gateway II"){kind=icon}
 "SHIFT 3"){kind=icon}
 "Bloons Tower Defense 3"){kind=icon}