This achievement is NOT cumulative; you must have 300,000 acorns at one time. Acorns can be earned by helping Ladybugs, completing missions on the right hand side of the screen, and finishing tasks with your bugs. Finishing piles with your ants or flowers with your bees is going to be how you will earn a vast majority of your acorns needed. The large junk piles that take 24 hours and the orange flowers that take 48 hours give the most amounts of acorns. Keep raiding junk piles and collecting from flowers and this achievement will come over time. It is best to go for this achievement after you have the "Bug Breeder" achievement since you should have enough bugs to help you earn income faster by then. While grinding for this achievement, try not to spend any acorns except for when feeding your bugs. This will help you earn the achievement quicker since you would not be buying anything unnecessary.