I did it!
Zero skulls.
It's very hard.
Actually, there is a glitch. But even with the glitch, getting 1 skull is pretty hard yet alone getting to 0 skulls.
The bug: The amount of seeds you get when chopping the tree depends on the selected tree, not the planted tree. I've noticed some issues with the amount of seeds you get and finally understood the reason. You can essentially get infinite seeds. Plant tree1, select tree2, chop tree and you get 2 seeds. But it's still very slow which makes it very hard to defend from the first fast monsters. Putting 4 Medora towers at the end spots might still be not enough to kill them in time.
P.S. The tree growth speed feels inconsistent. Some of the first trees grow almost immediately after being planted. Some take a lot of time.