Do you mean final boss fight or final level as in the whole city ? I didn't managed to be totally stealth, but i managed not to kill anyone, and it was hard, but not impossible in last part of the final level. Even in the last boss fight it was not impossible.
Anyone know if the new DLC has 'Boss Fights'? If it does I am not going to bother. Also, if your most recent save is near the end of the main game, does the DLC make any sense (i.e is is played as part of the main story or is it completely separate)?
There is one boss at the end but it was designed by the in house dev team and you can stealth past him or talk your way out of the fight.