The last perk should not be picked until the player can clear the goo in an area (this way you keep 1 slime and clear everything before each wave, keeping the forest at 100% purity). Doing so means this is a countdown until the player gets bored.
I think the shovel should not have been included to match the theme. At some point the player is too powerful to fear any wave. And the last 2 perks are clearly useless.
In a nutshell, not a challenge but it was fun and pretty to look at though.