I found the easiest way to get the achievements for "world's greatest boss" was to use children. since you need more of them and they dehydrate faster, they are excellent for this as the achievement requires refreshing employees. Simply do some of the earlier levels with only children and I got all three tiers of them very quickly.
Some would say that farming dehydrated children should earn more points toward evil but what do they know.
 "Bloons TD 5"){kind=icon}
 "Flight"){kind=icon}
 "World Idle"){kind=icon}
 "Sweatshop"){kind=icon}