Based on your videos, it sounds like you're asking for advice about UV AoMr (or Marksman Gold), which is a very different animal from the Marksman Silver. Since I have the liberty to see your play in action, allow me to comment on your actions:
- You play out in the open way too much. Now this isn't AS much of a problem if you're aiming for MGK, but before then you cannot allow yourself to stay unshielded by walls. Your main assumption when dealing with enemies that can't be taken down quickly is that you get their attention, then corner-shoot the crap out of them. In Hell's Arena, an ideal setup is one that has a pillar near the middle but off to the left or right, giving you a great position to snipe off Cacos/HKs/Barons as they approach. (The Demons and Lost Souls shouldn't be an issue if you have at least SoG(2) and a pistol in the prepped slot.) Overall, try to find a place that allows you to corner-shoot and use it as a base of operations from which you lure enemies and take them out.
- In Hell's Arena: your basic idea is good, but consider the fact that everything besides the Demons and Lost Souls still bunch up near the middle. For the first two waves, I usually find a safe spot near the edge, run-wait a few times (at 20s per run), then deal with the Cacos knowing where they are. It should be noted that Barons seek out ammo, so the third wave can be more hectic (as their AI will be unpredictable due to the scattered ammo).
- In Chained Court (with AM): The first half is basically what I do. It's risky because of all the captains, but positioning yourself correctly, as well as avoiding to enter too deeply into the left side of the map, can make the damage bearable. Aiming for the AM staff immediately after to wipe the rest is my plan as well. However, once these enemies are taken care of, you should make your way back to the spawn point, mostly avoiding to fire at Barons along the way. Inevitably your berserk will run out (I only take 2/3s of the supply, but this is based on a very precise timetable and I wouldn't recommend it unless you're strapped for medpacks) and when it does you're very vulnerable to the Baron's attacks. It's best to kill them with an easy retreat in mind, then revert to a more cautious strategy once the red disappears.
- In Hellgate: I'm surprised how well it worked for you to kill the first wave while in the center like that: personally I try to take cover off to the sides, as limited as it is there. Much of the difficulty can come from getting the Viles to come out, but once they do it's pretty simple. Your plan for the Bruisers was good but lacked proper execution. You want them to reach the bottleneck, at which point you can snipe them fairly safely, but if you're in a position where they can attack back, retreat until they're out of your vision, then attack where you know they will be. For the second Bruiser, simply retreat to the first room, spam bullets to the right (since his AI forces him to move directly toward you) and, if he's not dead by the time he's in your vision, move up/down a tile and corner-shoot the rest: you'll take a lot less damage that way.
Definitely pick SoG -> SoG -> SoG/DG -> DG/SoG as your first four traits. I tend to take EE as the fifth, but ONLY after getting those (or possibly fourth if I'm doing a single-pistol build for who knows why). Both DG and SoG(3) are vital early on, and you're not often shooting at the very peak of your range when it's not corner-shooting. Personally I take SoG(3) before DG: since tehtmi gave his reasons for DG before SoG(3), I'll explain mine as well:
1. The difference in DPS isn't noticeable, true, but the firing speed is. Whereas DG with SoG(2) will take 0.72 seconds, SoG(3) with a single gun is only 0.40s. In addition to a little bit more damage per shot, you're very nearly getting the same firing time, AND you can decide to move in between those shots. With a 0.40s firing time you will very often fire a shot without the enemy sending their reply, and so hit-and-run is a greater possibility.
2. You can still reload every twelve shots firing a single pistol as long as you have a pistol in the prepped slot to swap. This takes 0.8s, less than reloading and faster than any enemy that would take more than six shots at that point in the game. Since you should mostly avoid open-space battles before MGK (and hence before DG) spending this time rarely becomes risky.
Of course, having BOTH is just too awesome not to have by clvl4. If you're having difficulty hitting opponents early-game, make sure they're within six tiles instead of being at the very edge of your vision, which will improve your odds rather well.
After those traits, you have a few options.
- Some people will go straight for MGK, although this sacrifices offensive power for a boost in movespeed, autoreload (which, if you have speedloaders, isn't THAT great) and free shots when you dodge. The last bit is only significant against enemies that have dodgeable attacks, and the major enemies for which this matters are HKs, Cacos, and Barons. Of course, after clvl9 you'll have the opportunity to aim for other traits like Iro or Fin -> WK, but I personally believe that you want to build your pistol's strength before you reach the particularly nasty scenarios.
- Others will aim for Finesse and evolve into WK, readying themselves for super-modded pistols. This has the flexibility of eventually becoming MGK if you want, and it will make the best use out of finding any mods early on. Unfortunately, this also relies somewhat on getting the mods you actually want, as well as if you want anything other than speedloader pistols. My last UV AoMr win took this route and held out for WK(2) in order to create power-modded speedloaders, which ended up being powerful enough to plow through most of the rest of the game.
- A third pistol route is to go directly for power, getting SoB(3) and Fin(2) in some combination. I used to like this build a lot, since you'll reach the point of knocking back enemies quite reliably without having to mod your weapons at all. However, the knockback also can cause ammo to be wasted (as a result of a difference between bullet path and enemy knockback direction) leading to ammo issues, and so this build is best for single-pistol games, which don't often appear in a UV run. (Naturally you can just remove the prepped pistol if you're having ammo issues, but a UV player should plan for every trait to count as much as possible.) MGK is blocked, but HR(2) -> DM is not, so you can still get that desirable dodge against the Cyberdemon if XP allows.
I don't think MGK is really good enough to block SoB entirely, so I usually take Fin -> Fin and then decide between WK and SoB, filling in with Iro and eventually HR/DM over time. Some people like to take TaN but, given that my pistol strategies tend to revolve around conservative player, I find protection to be disregarded enough that I would prefer no armor to red most of the time.
If you're wondering how to go about killing the shambler, start the fight by pulling the lever and immediately shooting to your left: it's a straight path to the shambler itself and so you'll deal some decent damage before it has the opportunity to teleport. Keep yourself along that middle row, preferably near doorways, shoot and corner-shoot when it appears, and hide by entering the room toward the shambler once it's in melee range. It takes a LOT of kills but will go down in due time.
Some general tips to keep in mind (although I bet most if not all are things you know):
- Given the firing speed of pistols, you can often get in "free shots" against enemies. Use this to properly lure an otherwise-pesky monster toward you, where it can easily be picked off.
- When dualgunning, always reload your pistols one at a time. This means reload [r] followed by dualreload [R]. This is the safest way to go about reloading, since there's no time loss and it gives you the opportunity to take a look at the situation and whether or not you have the time to reload again.
- The alternate fire is basically used to pick off formers that might be carrying med-packs early on and nothing else. +3 accuracy for double firing time is usually not worth the trade-off, and once you have EE(1) accuracy should not become an issue.
- Since they are fairly accurate at a distance, using pistols to radar-shoot for enemies in places like Arach caves or mazes can easily save you from taking damage.
Hopefully that should clear up a lot of confusion regarding strategy and tactics for the Marksman Gold. It looks like you've already pulled a great effort out of what you're already doing right, so a few tweaks here and there produce the victory you're looking for.