XCOM 2

A SPARK is a special type of unit you can only get if you've installed the Shen´s Last Gift DLC. If you enable the DLC mission at the beginning of your playthrough you will get one for free when you complete the mission, otherwise it can be built at the Proving Grounds after complting the Mechanized Warfare research project.

Champion is the seventh and last rank a SPARK can get to. Soldiers gain experience by defeating enemies in combat or, to a much lesser degree, by being in the same mission when a squadmate defeats an enemy. So just be sure to take your SPARK in as many missions as possible and try to have them deal the final blow to enemies.

You will be able to choose an upgrade with every rank gained. Personally I think the best upgrades for this achievement are:
-Adaptive Aim.
-Strike.
-Wrecking Ball.
-Bombard.
-Channeling Field.

Remember SPARK´s are immune to fire and poison damage and can climb vertical distances without a climbing aid (ladders, pipes, etc).

Finally, SPARK´s don´t heal the same as conventional units. You will have to put them in engineering between missions, you can only repair one at a time.