Glimmer was only pretending to be weak. She pretended to not know how to use a bow or to be a shallow stupid girl.
In the reality she was an experienced killer, she used the strategy just like Johanna, to get overlooked so people think she is weak.

She didn't die in the tracker jack attack, instead she covered herself with District 4 female's body to escape.

Later when after only Cato, Katniss, Peeta, Foxface and Glimmer where left she slowly started showing her true side.

Foxface killed herself so there were four tributes left. Glimmer easily killed Cato and then started searching for Katniss and Peeta.

The District 12 lovers were in the middle of the forest (Katniss didn't have a bow couse Glimmer did)
She was sitting on the tree and from there shot Peeta in the back of the head.
Katniss was shocked to see Glimmer be able to climb a tree or shot from that distance. However Glimmer also shot Katniss making herself the victor.