HARD badge spent me three days until I found some tips :
(1)Complete wave 1
(2)SAVE game
(3)END game
(4)Start any level with sandbox mode
(5)END game
(6)Resume game, and you will earn money with wave 1 again
(7)SAVE and just repeat step 2-6
Some banana farm will be good, if u think these will help you put a + for this comment
 "Bloons Tower Defense 4"){kind=icon}