You mean the one in the office at the end of zone 5?
I'm stuck there as well, but from my attempts I can say:
- The shotgun deals much better damage than the rifle or the grenade launcher
- You need to save all your shotgun ammo for the bossfight
- You can stun him with the flashbang item, but it only works once
- You need to be at least level 60, probably higher
- You need to grind for the "Bread Carrying" skill so that you can carry more healing items
Bro i beat him second try xD
I was lvl 80≈ when finished.
Needed slightly better strategy.
All you have to do is use AR bullets on humans. (need 120minimum)
Chuck some grenades if they can fly far away.
Use grenade launcher on knights, also flashbangs but save 1 for boss.
You should save max shotgun ammo for boss and just use skills strategically to slow him down.
You should beat him with only shotgun, start to finish.