When you have all the apples, beat level 20 and after the cutscene you will be transferred to the boss.
Once you unlock this level, you will be able to choose a boss fight directly from the menu.
Viewing post in Leaftek and elemental power comments
There is a hidden room in the upper left corner. You can get there by air. Burn the bridge with the power of air over it. But do it in such a way that, with the help of fire, you can jump up again and take the power of the earth. While jumping down again, drop the statue onto the button and take the power of air. Quickly jump on the elevator. Thanks for playing and good luck!
Stand before the bridge with the power of fire, looking at the power of air. Use fire to burn the bridge and by bouncing off the walls you will be able to reach the earth's power in time. Or Stand in front of the stone's power looking to the left, use the fire's power and jump to land on the bridge, as you fire, jump up the wall, quickly take the stone's power and drop the statue on the button and collect the air power.