The Watchtower of Rustoria - The Chronicles of Omicron, #2

Omicron is a planet where the humans are the invading aliens. Corin is a native of Omicron, an aquatic assassin with a fantastical water version of asthma. She could be in the middle of ending a target only to wake up minutes later, having fainted. When she is sent on her final contract, she discovers it is quite possibly the biggest contract of her life: to assassinate every elite member of the Hand.
Joining her are an ancient graffiti-covered robot, a know-it-all little girl, and a brilliant mechanist with mechanical legs .