How to get past Wilhelm in "A Train to Catch" during Playthrough 2 of Borderlands 2 on the PS3?

First of all, you might want to reconsider your build. My experience with Zer0 is that his builds are highly situational, and using the wrong one in certain situations can make you feel very under-powered. I used his left (sniping) tree for the majority of playthrough 1, but after getting to the second playthrough it started feeling underwhelming-- I'm now back to it, but the other builds served me better in some situations. So, consider heading to a New-U station and re-speccing. I've found that his kunai (last skill in the center tree) can be devastating against certain enemies.

Second, you might want to grab some new guns. Guns start to decay in usefulness very quickly after playthrough 1, so if you've been using the same gun for a few levels now it's probably time to replace it. Grind a bit, or spend a few of your Golden Keys in Sanctuary. The difference will astound you.

Third, take out his surveyors first, always. They are a massive pain in the butt and trying to kill Wilhelm with them healing him is an uphill battle. But leave the other bots he spawns if they're not a huge nuisance-- you can use them for second winds.

Lastly, use your elements. Shock is THE best element overall, since it has no damage penalty against any kind of enemy and in playthrough 2 deals double damage against shields. I recommend a shock sniper for Zer0-- he'll dominate with it. Apart from that, definitely bring a high DPS corrosive gun, and slag, slag, slag. Zer0's kunai can apply slag, but if you're not using that build I'd recommend getting some slag grenades-- that way, you'll be able to slag him without switching guns.

Wilhelm is weak to corrosive weapons - IIRC, when I faced him the second time I knew this and he died to a corrosive SMG in less than a minute.

If you do Tina's "You are Cordially Invited" then you should have a weapon called the Teapot or some such, it works quite well against him.

I finally beat Wilhelm in Playthrough 2! A big thanks and +1 to everyone that provided answers. I'm answering my own question only because comments don't allow enough space for the following details. I ended up not re-spec'ing my character, and stayed as Zero just to see if switching my fighting strategy would help. With the tips people gave me, it did! In the end, combining slag/corrosive elementals were the key. Plus, I'm sure, just plain luck!

The strategy I used was to start with a slag pistol, then switch to corrosive SMG to whittle down his health a bit. Since the slag effect doesn't last long on Wilhelm, you have to repeat this. And when surveyors would spawn to heal him, I'd switch to my high-power, quick-reload shotgun to take them out in single blasts. There would usually be at least four that would spawn.

I found I could duck behind the ice stack at the far end of the plateau next to the burning barrel, to shield from his laser blast and missile launches. Even then, I got into sudden death several times, and luckily had a surveyor nearby to get a second wind. I would run in a clockwise circle around Wilhelm, throwing out my assassin decoy as often as possible to misdirect his attention to get in close for a quick shotgun blast or more while he was slagged.

Finally, when his health got below a quarter, I used slag grenades instead of the pistol. That let me focus on just two weapons, the shotgun and SMG. In the end, I used 5 of 8 grenades, being very careful to throw one only when the slag effect had just worn off -- I didn't want to waste any grenades.

Using this strategy, I still died a few times, and it took me 45 minutes of game play to beat Wilhelm. But I did it, and my frustration with the game has subsided a bit. The encouragement from everyone helped a lot, and I'm ready to continue PT2, knowing that this isn't even the hardest map! :-/

Here are some details of my skill level and weapons, in case it helps anyone else.

Character Level 42, Shield 8719, Health 19342

Weapon                      Lvl Damage    Accuracy Fire Rate Reload Mag Size Other
--------------------------  --- --------- -------- --------- ------ -------- ----------------------
Jakobs Handy Coach Gun      40  1566 x 13   42.1     14.5      2.3     2     +11% dmg
  Shotgun (green)                                                            2 ammo per shot
Hyperion Amplified Impact   37  1500        97.6      4.6      1.9    20     SLAG 18% chance
  Pistol (blue)                                                              Firing increases accuracy
Bandit Bulets Go Fasterifed 37  646         90.4      8.6      3.5    50     CORRODE 11.5% chance
  Barfy SMG (green)                                                          846.6 corrosive dmg/sec

Hyperion Rubberized Slag    36  11890       Radius 464, Fuse Time 0.1
  Singularity Grenade (green)

Eridian Relic (white)       35  Corrosive Damage +13%
Bandit Survivor Class Mod   39  Max Health +1767, Health Regen 47.9
  Assassin (green)
Anshin Hippocratic Adaptive 37  Max Health +3011
  Shield (blue)

The key to killing Wilhelm is to shoot him in the face when he is creating his Shield Surveyors. It destroys the surveyors and causes him to stop making them. Kill all the gun loaders then keep moving in circles. I did it with 17th level Zero, a shotgun, and a corrosive sniper.