you need to rush the whole pyro mage tree asap for the flamethrower build, with splinter and rubber bullets being focused next, and flamethrower scarlett can clear end wave without moving her mouse (aka a fraction of what crossbow can do)
I usually prio: burn>bounce>scatter>vit+giant>fusillade>run&gun(pick up 25% atk speed asap though, helps early) using scarlett, infinite healing and damage in one combo with some of the highest consistency I've ever seen.