here's an idea:if you save the game before the last round and you beat the last round, u can go back to menu, then main menu, then click resume game. You will not have your career rank exp go down(the number of balloons to pop to go up a rank) so it is quick and easy "exp" u just keep repeating and u will lvl up quickly
 "Parallel Kingdom"){kind=icon}
 "Frontier"){kind=icon}
 "Bloons Tower Defense 4"){kind=icon}