This is my strategy so I don’t land on a level that is so difficult I cannot move forward and am stuck slow farming to buy upgrades.
Build your defense to the point that you can auto-play with 2x speed and big enemies (kitty cat selected). Then when you get to a level that you cannot pass, spend your money on upgrades and try again. If you still cannot pass, slow the level to 1x speed and if necessary use regular sized enemies and you should be able to pass it, chances are this will be a boss level and those adjustments will get your through it. Now you need to farm until you can clear the levels 2x speed and big enemies once again.
This is important to not get stuck on the Island level, once you get past lvl 500 on island you can go to the forest and if for some reason you get stuck on a level on forest you cannot pass at 1x speed and regular sized enemies (probably because you stopped following my advice), you can always go back to the island and fast farm a level there.
If you find yourself completely stuck you should rebirth, you will start from scratch but be given rebirth coins to spend in the rebirth store. This will give you powers from the start and make it easier to progress through the stages.
Ideally if you are going to farm for any period of time, say overnight while you are sleep. It is best to be able to pass the level at 2x speed and with big enemies for max farming returns. Remember to ensure you have the farming button selected as well as the auto button. I don’t use the AFK button, other than saving some of your computers graphic resources, I’m sure of what it does.