I have currently fought in Uber Trist for about 5 times now, and I beat them all with ofcourse many other ppl's help.
I find the best strategy to be this, and you can finish the final Trist within 30 min if you have 1 of;
1 Nec (Summon preferred)
1 WW Barb (BvB preferred)
This is the smallest # of people, 3, and still be able to finish Trist quick enough. Hammerdins strangely seem to suck crap so very much in Trist, when they're the kings of PvM. They do somewhat good in clearing minions though. Ofcourse you probably will have different set of characters, so try to include Lightning Trappers and Cold Sorcs (FO preferred over Blizz). FoHers are highly uneffective unless there are lots of elemental chars in the game, zons could be very preferred if standing back to back with the River or wall and spamming Multi's to take out minions. Windies provide little help except for their Oak and ability to ignore Meph's Conviction although their Cyclone Armor gets taken out within like 2 hits.
First some basic facts of final trist. You can't cast a TP in it anywhere, antime. Baal < Diablo < Meph is the order of their strength. They each summon different kinds of their minions, for example Baal seems to summon Pit Lords and uch while Meph summons the Undead. In Trist, if you don't take care of the minions constantly, the whole map will the packed with them. You will see Skeleton Archer's arrows everywhere, and they hurt more than Hell Baal's attacks. I would bet that all 3 monsters are either completely immune or 95% resistant to all elemental, perhaps 95% immune to magic (unlike DClone who was only 50%) and about 50~90% physical. These guys' life regenerate crazy, and Prevent Monsteal Heal is useless on Mercs since they go down easier than twigs.
When you open the portal, have everyone freshly BO'ed from the Barb, in Worldstone wp or etc.
Everyone goes in at the same time, and as soon as Nec goes in, he has to Life Tap like CRAZY. I mean it. Like he's gotta Life Tap the entire area that is within 2 screen radius of the red TP. WW + Smiters have to try their best leading Meph and Diablo away to the other corners away from the red tp, and this is best done by Barb + Nec who have teleport. Expect to die once they're right in the corner, and just rejoin. So Diablo + Meph is away for the moment, do not go ANYWHERE near them, and concentrate on Baal.
Nec should constantly cast Life Tap everywhere since more and more minions come to life from the ground, and I mean like holding down the right mouse button. Reviving everything that's dead is also good idea of drawing out the fire from other minions, or Bonewall does the trick too. Baal should go down easy. Any other chars that's not mentioned shouldn't attack Baal but minions as they do tremendous damage. Barbs + Melee Palys keeping their focus on Baal only, while other ppl in party cast hammerfields + trap lines + spamming their attack in the general communities of the Tristram. (Zons should NEVER EVER fire their arrows toward Diablo or Meph, as they will run toward your group at 500mph.)
When you have fought em all, everyone in the party has probably died at least 2 times except the ones going for Baal with Life Tap. You should not lose focus of which one is the real Baal because he will summon more than 2 of himself if you stall while his health is very low. If the red portal becomes clustered with minions preventing the Nec to get his bod, he must rejoin immediately because although he can still Life Tap the duration + radius of it is too small to be effective.
When you have taken down Baal, it is time for Diablo. He's a bit harder than Baal, but just make sure you guys have Life Tap. I can't stress how useful Life Tap becomes. Everyone should try to take heat off the melee guys attacking till theirEBotdZ's (NL, no Grief) get broken literally because they seriously need to hit over 1000 times to take down Diablo alone.
Once Diablo is down, clear any minions nearby the red portal, and everyone should go in, and heal + get fresh bo from Barb. Buying pots + Repairing is out of question since everyone should have empty exp bar with 1~4 gold. Except the melee guys. Once everyone is refreshed temporarily from good ole Malah, go in for Meph.
This guy, is hard. I mean it. He's harder than Baal + Diablo combined. He has the lowest health, but with truly insane Lightning damage + self equipped Conviction, he shredded my Nec who was using 4 PTopazed Shield + GA + TGods + Dual Wisps + 3 Ort Helm within 5 hits. Not to mention this guy has more minions than Baal or Diablo. He also has extremely high def, so it is very very hard to hit this guy. If there is a Hammerdin, ask him to turn his aura to Blessed Aim instead of Concentration. If not, this is where Smiters truly shine. Ignore monster Def allows him to always hit Meph, and with Life Tap he should be leeching fast enough to not die. Everyone probably will be surrounding the melee people + Meph, and they will be surrounded by cluster of minions. If anyone dies, they should rejoin because it is impossible to get body since it is surrounded by 100 archers. Make sure Life Tap. If everyone dies and only the melee dudes are left, they will still die even though Life Tap remains. If no one is in Tristram and Meph lives because everyone rejoined to get bod, you will be more than lucky if he hasn't recovered fully.
It seems that once only couple minions remain, just ignore them and everyone should pinpoint their firepower into Meph. Meph constantly casts Charged Bolts, and they hurt like hell while managing to hit almost everyone with it. Once he gets to very low health, everyone will gather right up around him even including Bowazons because they're so eager for the Hellfire Torch LC.
After you kill him, you're more than likely to lag as Meph's death scene is quite stunning in terms of graphics and require much qualith to be lag free, but he drops the Unique Large Charm along with same number of Standard of Heroes sc's as the number of ppl in game.
You probably would have died many times, and everyone else has more like, but Hellfire is worth all that.
+3 certain char skill, could be pal, could be zon, could be any other.
10~20 all attributes
10~20 all resistanes
25% chance to cast lvl 30 Firestorm when striking
I already got 2, a Nec and a Pal one. Those are pretty insane.
Good luck on all of us with our future battles with the final 3.
Post your own strategies when you fought them yourselves.
I will post couple screenshots later on about the guide. btw, Diablo Clone is nothing compared to this.