This Guide Is Only For the Boss Fight with Barnsby:
This is definitely the hardest section of the game in Very Hard. If you made it past the brutal first 2 levels from the Civil War and felt that the game was a breeze afterwards, this is a final rude awakening. However, a few tips will make this fight MUCH more manageable.
1) Play as Ray with the Superb Volcanic Pistols (2), Superb Classic Rifle and a full stock of Dynamite.
2) Be sure to kill all of Barnsby's men on the ground level BEFORE triggering the checkpoint going up the spiral staircase. Thomas will babble on about getting buried in sand but feel free to take your time and ignore him as the sand cannot kill you. This should leave you with only one person on your way up the spiral staircase after the checkpoint making life a lot easier when you are re-doing this several dozen times.
3) When you get to the top of the spiral staircase with Thomas, HANG BACK! Don't trigger the bridge to fall but instead shoot Barnsby from afar (easier with the rifle) so that he is one less thing to worry about. If you do not trigger the bridge collapse his men will not spawn, leaving only him to deal with! You know that you have damaged him enough when he eventually makes a pained noise, stops talking to you, and stops firing at you.
4) Trigger the bridge to fall with a full dynamite stock in hand. As soon as you see Barnsbys men start to appear, throw the dynamite continuously every few seconds so that it lands in the area just behind the pillar with Marissa. If you aim well with good timing, you will take out the majority of his men, usually leaving the just last wave of snipers and a straggler or two. I found that aiming up close to the ceiling (well over Marissa's head) and just to the right of the pillar managed to land it in a spot that killed 8-10 of his men.
5) For me this step was crucial to keep myself from getting destroyed every time by the snipers: TURN OFF DYNAMIC COVER!!!! This will allow you to creep around the pillar without accidentally moving too far into the open and getting instantly murdered by the snipers.
I found that creeping around the left side of the pillar was easier for whatever reason. Make sure you only peek around the pillar EXTREMELY slowly! If only the snipers in the back are alive, only peek out immediately after they fire a shot so that they are caught reloading.
You know that you have killed the last of his men when Barnsby starts getting chatty again and challenges you to a duel.
Unfortunately even if you do everything right, you will die over and over from getting one-shotted by the inhumanly quick and accurate snipers, or have dynamite blow you to pieces, but with enough retries and some luck/skill this is manageable!