I lent my iPad to an 8 year old to play Boons. He got to Level 40, and then I picked it up from there. In the map associated to thsi article is the approximiate build / upgrade order.

General strategy - put down units that knock out monkey at the start and midpoint (the monkey appretice!). This works really well in this particular map b/c it gives you several opportunities as bloons cycle around. Second major strategy is setup for the Cammo run and the Ceramic run. This is done w/ the dart monkeys upgraded to 4/2, 3/2 and the monkey engineer upgraded to 4/2 (very important). Also, get a pair of subs down fairly early (2/0) and one dart on the bottom wall (0/2). This will allow the subs to attack anywhere you have a tower.