Fight with Fetch is first "real" boss fight in the game. You'll have to face her during the main quest in Seattle - Go Fetch.
The fight with Fetch starts in the theater. At the beginning try to familiarize yourself with her two standard attacks. She will try to deal you damage with a laser and you can avoid it by using Light Speed and by hiding behind covers.
Do not try to approach Fetch because she could then use energy explosion and such attack can strongly weaken or even kill you (if you are already wounded). It is worth mentioning that you do not have to fight with her in a melee to win this duel, so keep your distance from her.
The fight with Fetch is divided into two parts. In the first one you have to absorb energy from nine neons located in the building (this is also a way to refill Neon energy). Each time you approach neon position yourself in such place that Fetch can't hit you and then start absorbing energy (press touchpad).
Clear out the entire ground floor, finding all neons and absorbing their energy. But this is not all, because there are some neons on upper balcony. To reach them run over walls, watching out for Fetch. When you absorb energy from all neons, the second part of the fight begins.
In the second part of fight you'll be able to use basic distant attack of Neon power. It is Neon Beam which can be activated by R2 button (it is equivalent of Smoke Shot from smoke attacks category). Start using this attack to weaken Fetch but be aware that she will also use it, so keep moving and/or use covers. The girl can hide from time to time and in such situations track her down by following her traces.