Heya,
Well, the way I see it, there's more to the game than swinging a weapon or shooting stuff. Lots more. I bet that strong Militant has tons to do, and is having so much fun, when he's not just rolling and killing stuff.
Make the campaign more diverse, so that absolutely narrow characters are not as useful/fun, and promote people spanning out their purposes. Do more investigative sessions, less combat sessions sometimes. Change it up.
And when it does come down to combat, see how well the Militant's willpower holds to something that affects or controls things via opposed willpower tests. Let him come up on a powerful psyker, or even an evil Navigator or something. It will get ugly.
Very best,