Uber Tristram
Uber Tristram is a special zone that can be accessed only during the Pandemonium Event. It is an exact copy of the destroyed Tristram that is found in Act I.
In this version of the town, high leveled players must take up the task of defeating the combined forces of the all-powerful Prime Evils: Über Baal, Über Mephisto, and Pandemonium Diablo.
Background
Note: considering that the Pandemonium Event is a Battle.net only quest and is not available in the Single player or even open Battle.net games, it is generally not considered to be a part of official lore.
"After her legions of Succubi and strongest daughter Andariel were killed by mortals who successfully defended the Worldstone, Lilith, the Queen of the Succubi and matron of all demons, swore to personally intervene. She masterminded the revival of Diablo by manifesting herself in the mortal realm and performing a ritual of Dark Summoning. This ritual, long thought to be impossible by the most powerful mortal wizards, allowed Lilith to link with Diablo across realms, strengthening him until he was finally able to once again reincarnate himself in the mortal plane. Rage and a deep yearning for revenge boiled within the eyes of the Lord of Terror, and together he and Lilith resurrected the two remaining Prime Evils as well as Duriel and Izual, two of their strongest agents.
Diablo announced his grand plan - the Three were finally strong enough to spread Pandemonium throughout Sanctuary. The Three amassed their forces in a ravaged outpost within the mortal realm - Uber Tristram. It is only here that heroes can hope to best the Three once more, and save Sanctuary from certain destruction."

— Lilith's description

One simple way to kill all of the three Übers in particular, is with the Smiter Paladin build.
A fool-proof way to kill the Übers is to run straight up when you enter the portal. Then go around to the left and go halfway down. Then slowly walk toward the center of Tristram. This causes Diablo and Baal to charge at you. Run away to the left and top again so Mephisto doesn't instantly kill you. Then work on Diablo first because his Pit Lords can kill you pretty quickly. Baal comes next. It may take a few tries, but make sure you make your game permanent, so if you die, you can save and exit and come back. After the two easiest are dead, Mephisto is left. He is easily the hardest monster in the entire game, though there are a few different ways to kill him.
Depending on your skill as a player, the speed of your computer, and the ability of your build. The first way to kill him is to just charge in and attack like a madman. If Life Tap isn't cast within the first 10-15 seconds, you will die and have to start over again, though by going to the various magic vendors (such as Akara in Act I) you can buy a Wand with charges of Life Tap. Simply by having the wand in your second weapon set allows you to cast Life Tap before going in, making all of the three bosses trivial and eliminating the luck factor involved with other methods. The second way to kill him is to trigger him, then run into a building away from the portal, and let him follow you into it and get stuck. Then, either teleport out (if you're wearing Enigma), die, or save and exit. Then, after you're back in the game, run straight to the house and attack him. Since he is kept away from his minions, his Conviction Aura is less of a threat, though this may take a few tries.
Build Overview for Smiter Paladin
Smiter Paladins get their Build name from the main skill you'll be using to take down Diablo's hordes. Smite is one of the best skills in the game due to its stunning ability, and a properly built Smiter Paladin can single-handedly slay their way through even the toughest challenges in Diablo II: Resurrected. Other options for Paladin builds include the Hammerdin, which uses Blessed Hammer to deal strong AOE damage and clear enemies quickly.
It should be noted that this is a highly specialized build that's focused almost solely on fighting Uber bosses in the Pandemonium Event. This is probably not the build to use when leveling. It is possible to build a Smiter with relatively inexpensive equipment, but do note that this is a very popular build in Diablo II: Resurrected. This is because killing Ubers to farm Hellfire Torches can be very profitable. If you're trying to min-max your equipment, things can get pricey.
Skills for Smiter Paladin
The Paladin has access to three Skill Trees, Combat Skills, Offensive Auras, Defensive Auras. The bulk of your required points will go into Combat Skills, but you'll also need to invest several points into Offensive Auras to get Fanaticism. This will leave you with several points left over, giving you a few options. For lower-budget players, we recommend maxing out the Conviction Aura to counter Uber Mephisto. However, players with better gear may want to put points into Resist Fire, Resist Cold, and Resist Lightning Auras to get that extra boost. However, you can alternatively max out Sacrifice and Zeal to make your Smiter a bit more versatile.

Best Charms for Smiter Paladin
Small Charms
Smiter Paladins looking for Small Charms should consider looking for ones that boost All Resistance. These can help out in a pinch against the Uber bosses. Annihilus is the best Small Charm you can have in your inventory, granting +1 Skills, +Attributes, All Resistances +10-20, and even a Bonus Experience modifier, making it very valuable.
Large Charms
Outside of one major caveat, most Large Charms aren't worth the inventory space. However having a Hellfire Torch, which grants +Skills, +Attributes, and All Resistances +10-20, is a must.
Grand Charms
Look for Skiller Grand Charms. You want to find a few that give +1 to Combat Skills to boost your Smite and Holy Shield as much as possible.

Best Equipment for Smiter Paladin

Weapon
For Weapons, Smiter Paladins have a few options. The two you'll see most often are the Grief and Last Wish runewords, or Heaven's Light(+3 Skill and 2 Socketed). Grief is an excellent option because of the way its damage stacks. This effectively makes Smite deals way more damage, turning into a true powerhouse. Make sure you place the runeword into a Phase Blade to get those extra Attack Speed buffs. Last Wish, on the other hand, gives players a source of Life Tap, which is incredibly important to this build. That said, you can also get it a bit more easily with the Dracul's Grasp gloves, so Grief might make more sense for you.
Players on a lower-end budget should consider the Heaven's Light(+3 Skill and 2 Socketed).While not as powerful as the other two, it does provide a nice bonus to Crushing Blows. Plus, it won't cost you nearly as much.

Shield
For your shield, Smiter Paladins need to look for either the Exile runeword or the Herald of Zakarum. Exile gives you Life Tap if you're not using Dracul's Grasp gloves, while the Herald of Zakarum has a bigger boost to Smite damage, higher resistances, and the potential of an extra socket. Personally, we'd recommend the HoZ, but you can't go wrong with either.

Helmet
Either Guillaume's Face or Crown of Ages are going to be your go-to helmets for a Smiter. Guillaume's Face is our preference of the two because it boosts Crushing Blow. If you're looking for a budget option, just find something with as much Crushing Blow as you can find.

Body Armor
Most Smiter Paladins will use the Chains of Honor runeword in their armor slot, though some do prefer the Teleport that the Enigma runeword brings.

Gloves
Smiters have two options in their glove slot. You either want Dracul's Grasp or Steelrends, depending on where you're planning to get your Life Tap from. If you're getting it from the Exile, go with Steelrends. Otherwise, get Dracul's Grasp.

Boots
For boots, your best option is Gore Rider. These increase Open Wounds and Crushing Blow, which are incredibly helpful against Uber Tristram bosses.

Amulet
In this slot, we like Mara's Kaleidoscope(Recommended 30 RES), which gives you boosts to both All Skills and All Resistances. Both of which come in handy for what the Smiter is set up to do.

Rings
Like many builds in Diablo II: Resurrected, the Smiter can get a lot of use out of Raven Frost. With your second ring, you have a few options. Most go with Bul-Kathos Wedding Ring, but others will go with a Rare Ring with health boosts.

Belts
Verdungo's Hearty Cord is a great all-around option because of how much it boosts Vitality and Life. Staying alive is important in Uber Tristram, so pick this up if you can.