For this type of game,

It's always helpful to prioritize skills that aid in clearing the field first. High damage is useless against sheer numbers, so splash damage and spread effects are the top priority. Max out bullet spread and penetration as quickly as possible, then slowly build up attack power while picking up skills like Meteor. This makes clearing trash mobs easy, and even if you're a bit short on damage against elites or bosses, a comfortable field provides ample space for kiting, making these skills essential from the start. Magazine capacity isn't bad either, but I think reload speed increase offers more value.