Zero Repeat Forever by G.S. Prendergast (Nahx Invasions #1)
Sixteen-year-old Raven is at summer camp when the terrifying armored Nahx invade. Isolated in the wilderness, Raven and her fellow campers can only stay put. Await rescue. Raven doesn’t like feeling helpless, but what choice does she have? Then a Nahx kills her boyfriend. Thrown together in a violent, unfamiliar world, Eighth and Raven should feel only hate and fear. But when Raven is injured, and Eighth deserts his unit, their survival comes to depend on trusting each other.