I've checked the leaderboard videos and noticed none of them used the quick-kill I've found for Mr. Tong a while ago. My video shows it with buffed health but I'm sure there's a viable way to get it without tanking it.

What you wanna do is get to the last part of the mission with the final three guys before reaching the sauna. Kill two of them and head to the sauna. Stand in front of the door and then drop your weapon (make sure it isn't clipping through the door, this may cause problems). Afterwards take out the final thug and then head back to the sauna.

After skipping the cutscene, head back to the entry door. Although it de-spawns the weapon currently in your hands, it does not get rid of any of the weapons from the previous gunfight. You can pick up the gun through the wall and use it to not only quick-kill the the brawler and grappler thugs, but also Mr. Tong. Note: the final blow on Mr. Tong has to be a counter.