Also, there seems to be a bug on the first lvl with two colors, I can't get them both to the objective properly, they just pass them and nothing happens. Another bug I've found (I really like this game, that's why I'm posting them) is that if you draw a line over your circle, that circle kinda teleports to some part of the line. Just trying to help here.
I see that some levels, the ink is limited, but there is a way to bypass that, just draw a big line quick, if you try to do something slower, the ink ammount will be really small.
You can beat the game easily (without a single hit on the Tree Of Life), just buy the weapon upgrade (not sure about the real name, just forgot), the one that increases the damage of the character. You won't need to add a single tower to beat the 30 lvls.