Do you have fully upgraded weapon?
On my non-lethal stealth playthrough, I put everything into my revolver and made it my boss killing weapon.
Basically, keep you distance as much as you can, and wait for him to start shooting from across to room. Rince and repeat. It worked at the highest level of difficulty for me. Another opening is when he throws grenades. You should be able to get a few shots off before moving away from the grenades
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