Firewalker (The Saga of Java Mountainstand Book 1)

For Java Mountainstand, the only way to avoid an arranged marriage to a brute is to run away from her life, her home, and him. Java joins the all-female mercenary company called Freeholm's Firewalkers where she is made a Scout. As a scout, Java finds herself in one of the most dangerous positions in the company--and the most exciting.

In response to Emperor Frander's attack on the duchy of Lender's Dale, the Firewalkers are sent to the border between Lender's Dale and the duchy of Greencastle to keep him out as well as they can.
Java and the scouts become the most active force against Frander's forces. Java distinguishes herself in many ways, including breaking the rules to rescue her sergeant's sister.

Java's fortunes turn decidedly for the better when she rescues three young magi, and then stumbles upon the last living member of the Lender's Dale royal family, Samantha.

Then things get interesting.