Since the video here isn't working I went on youtube and found another one. I got stuck on the final boss until watching this video*. Basically you want to use Ross for the final boss because of his extra health. You can hide on the back right corner and toss grenades at the boss. If you don't use grenades you need headshots which are very difficult with this boss. I tried several times with Bull and a couple with Ross. When I tried just using grenades I nailed it my second time. When you hit him with a grenade you will get a white flash on the screen and then he will switch sides. Since I was hiding on the right side of the room I would just run out quickly to scare him back to the other side so I could hit him with another grenade. I did that 4 times and game over.
*I do not take credit for the video, just wanted to share it
The share link would not work in the youtube link section so I listed the link separately.