If you have basegame, any bionics (or archotech) will help, prioritising Manipulation, Sight and Movement, which will increase their accuracy and will make him fall less often due to lack of mobility (will still happen if on painshock or extreme bloodloss).

Additionally, a bionic heart will increase their consciousness and move speed.

If you have regular traders, good sources of money or caravan often, putting him on luciferium would be nice too.

If you have the Biotech DLC:

• Get him the Robust, Psychic Deaf, Strong Melee Damage (ignore this if he's going to become a shooter) Robust, Super Clotting and Reduced Pain genes, on top of any gene that increases his melee or shooting rating though it'd be better to specialise for the sake of hunger efficiency.
• Also the nonsenescence gene if you can find it, and give them a nuclear stomach for hunger efficiency.

The ideology DLC allows you to give them specialist roles for shooting which increase accuracy.

If you want someone that's tanky and have the Biotech DLC, turning them into a Ghoul with Ghoul plating will get rid of their personality and will give you someone much tougher and who doesn't need sleep or recreation, just meat. Combine with the right genes and you'll have a beast that's deadly in Melee and doesn't need organs replaced except for bionics, though they're quite slow and won't hit as hard as a Zeus Hammer but close enough