An A-Rank Commander, but a Necromancer

I'll polish this blurb later. For now, I'm just jotting down an idea.

An academy story about a necromancer who ends up in a sci-fi world, uses mechs as skeletons to control the undead, and becomes invincible.

The female lead has A-rank mental power, but can defeat all kinds of opponents from S- to SSS-rank.

Theme: Don't define a person by their rank.