To Build a Better Soldier

From the introduction:

The Star Mecha are a completely mechanical alien race and in their ongoing secret war with Alien Destroyer, they have learned to make use of the resources that the Earth provides, in the form of the dominant organic life form. A highly belligerent race, the species trains the best of its kind as warriors but then discards any damaged ones and trains new ones, rather than upgrading the old models. The Star Mecha see such waste as illogical and repairs the damaged organisms before repurposing them to use in the Star Mecha's own war.