Shrapnel is maxed early to maximize it's damage potential before it becomes less effective than your normal attacks.
Take Aim is gotten in conjunction with Shrapnel to allow you to farm at a relatively safe distance.
Assassinate is gotten whenever possible. When used in conjunction with Orchid Malevolence, the damage is quite significant.
Headshot is a good skill with a minibash and damage bonus from the minibash, but for this build in particular, your main focus is on towers, so it's rather pointless to get it early.