Balancing Kog, the easy route
Step 1) Smooth out his Ult bonus damage. Gaining 2.667% bonus damage per 1% HP the target is missing would make it reach x2 damage at 25% HP while making the Ult vastly more useful at higher HP values.
Step 2) Add bonus AD scaling to his W OnHit. Adjust the bonus and base damage as needed to bring Kog up to par.
That's it.
That's all Koggy needs.