Bokurano: Ours, Vol. 2

Saving the world is hard.
Saving yourself is even harder.

Giant robots are invading Earth and fifteen teenagers must figure out a way to stop them. The key to mankind's survival is a towering mecha known as Zearth, but the iron giant's power is not infinite. It gets its strength from the power of life supplied by each teenage pilot. That's a deadly price to pay for someone not yet in high school.