Legacy of Bhaal difficulty, a guide to the game on hard.

There are many combos that are pretty solid and it depends on what your objective and difficulty is. Mainly you can focus on hitters, casters(mages) or a combination of the two. For example, the triple mage setup struggles on Legacy setting where hitters are required to overcome high hp of numerous NPCs. Also people trash Jan but they don't know his secrets. So lets run down my opinion of comps that can beat Legacy difficulty because the vanilla game is to easy and can be completed with multiple comps.

First of all, never do a double cleric/druid comp and avoid Aerie like the plague. I made the mistake of doing an evil play through with Hexxat, a druid, and Viconia, and it was painful at some points with only 2 hitters (Korgan, Dorn).

It's hard to work in double mage which I highly recommend for Legacy mode where some bosses are so beefy that it takes forever to kill them without instant death spells. I recently attempted Firkraag and he's so ridiculously rough prior to chapter 5 that I actually switched my strategy after about 15 reloads where my best attempt was about 40% and just went for instant kill strat with spell spam. Using Jan as a 2nd mage has a lot of drawbacks such as limited access to lvl 8 spells and never unlocking lvl 9 spells until extremely late in the game. The advantage is that Jan can use the simulacrum helm that normal mages cannot use (warrior and thief only, Nalia and Imoen excluded). This strat is only limited by his access to scrolls at which point he takes off and surpasses most of the mages except maybe Edwin.

Lets talk slots and party comp.
Tank (absorb or prevent damage, aggro usually based on distance)
DPS warrior (dual or 2handed) 2handed is superior on Legacy difficulty because aggro is based in part on distance and 2handers help keep DPS safe from massive hits. In clusters of enemies it's harder to keep your tank the closest and stray hits can occur on dual wielders.
Range warrior (or Archer)
Healer (support, heals late game, mostly support by buffing and curing party and summons early and mid game)
Mage (support, disables groups, hastes party, summons mid game, wands and strongest vs bosses)
Thief (traps, traps, locks, backstab). Strong option for multi-class.

Your second mage can share a slot with another warrior, healer, or thief. I recommend thief but there is a way around this. There is one combo I've never seen mentioned ever in any guide and it's the gnome cleric/rogue dual class. Most people hate thief duals, especially because of trap cheese but honestly you can cheese the vanilla game with numerous strats and traps don't work on massive clusters of NPCs such as Sendai enclave or Sendai fight herself so I'm honestly unsure how much cheese it is. Also getting Demigorgon to run over traps or one shotting Melissan on Legacy is very very hard if not impossible with traps so at best they just burn one of his summons or her heal spells. I personally like the gnome cleric/rogue combo because even though it can't backstab in EE, it does fill two trash roles and frees up a slot for another pure mage or warrior.

Last comment. For serious power gamers, the classic setup is a bunch of half orcs in varying degrees of multi-classes with a Sorceress or Edwin in there somewhere. The biggest disadvantage to multi and dual classes on LoB is thaco. I learned the hard way when my party fatigues while fighting a golem in Nalia's keep that thaco and ac is actually a huge deal. They were rolling 26s and still missing in part because of fatigue (due to haste) and they ended up only landing hits on natural 20s which taught me just how low ac gets on SoA and LoB. Jaheira and Anomen and their infrequency in landing hits is another great example of how important thaco is. This means on LoB, high strength (which contributes to thaco) still isn't sufficient.

Tanks
Korgan - Axes are awesome, high con, high saves, berserker kit is amazing, can ranged with +3 weapon. Can eventually dual wield Crom Faeyr.
Player char (Dwarf Defender) - Raw damage reduction. Can dual-wield on Legacy difficult, using flail in off hand. Can only 4 star in axes or warhammers which is honestly fine even with dual wielding.
Haer'dalis - Very difficult tank with lots of micro, hair trigger casting, and mastery of spell selection. Must be super careful vs magic damage which can randomly burst him. Also must be super careful around dispels such as Inquisitors who will remove stoneskin, mirror image, and ITMW. If you play him as a tank you can't use him as a caster because all his spells go into survival. Can do some crazy things with spell immunity and is a solid LoB tank from the get go but gets passed up but Korgan or Player char.
Player char (Berserk/mage dual) - Takes awhile to come off the ground. lvl 10 warrior has low thaco, lvl 13 is better but comes on-line super late. Eventually becomes possibly the strongest tank. Berserk = immune to everything, even stun (avoid spell casting if you think you need to use this to block something important like stun or level drain). My favorite is to go evil in hell and dual wield long swords.
Mazzy - High innate hp and dex for ac. She fills this role decently for the early to mid game but should eventually be replaced full time.
Jaheira - If you play her as a tank you need another healer because her spells all go into tank and damage mitigation spells. She's very weak vs spell damage.
Player char (Swashbuckler) - Can reach really really high ac. Haven't tested the effectiveness of -25 ac or better on LoB but combined or caused from Skald, defensive harmony, improved invis, and other gems(glitter dust and slow), could be top notch.

DPS war
Valygar - Amazing, has Katanas(stun), can scout, can backstab(up to x4), can reflect spells.
Kheldorn - Amazing, Carsomyr, dispels, truesight, destroys casters and probably the best dps because of kit. Can eventually learn 2 stars in axes for Azuredge or long swords for daystar (lvl 12).
Dorn - Very very strong kit. Can use many two-handed weapons including swords. Silver sword can vorpal and can switch to blackguard sword or a polearm once you get Sarevok. Dorn's kit is just solid vs casters, dominates big groups, and his special helm later makes him a god of immunities.
Haer'dalis - He should be reframed from being used because of how delicate he is on higher difficulties. Being dps means he can help with casting spells but this isn't that effective since his spells are useless vs Demigorgon and Melissan. He'll bring a few debuffs like glitterdust, slow, breach, lower magic resist, and the buff improved haste.
Player char (monk) - Any monk works including, even, Rasaad. Evil monks are more tank oriented but they're all just simply great. Their early to mid game is weakest and they only grow with level. They cap at 4 attacks per round instead of 6 with varying degrees of thaco per strike.
Sarevok - Hands down the best NPC in the game. I've never seen his special ability, random chance to do 200 bonus damage, but he seems to land vorpal with silver sword a heck of a whole lot while doing tons of damage. ToB was practically designed for him as he slices through hordes of generic NPCs. Silly not to pick him up.

Ranged
Player char (Archer) - Easily the strongest ranged class in the game. Go short bow, slings, or crossbows. Various strenghts and weaknesses but still all strong in different ways.
Player char (Wizard Slayer) - Must be good alignment. Virtually unheard of in guides but incredibly strong. Uses Azuredge axe. Weak vs self healing, cannot use ring of regen but can use the weaker ion stone. Cannot use most potions unlesss they heal or cure something. If second created char in a game, you can choose evil options in hell and still use Azuredge. I discourage dual classing a Wizard Slayer (make a dwarf or orc), because Wizard Slayers cannot use Int or Wis Potions. Because they suck at healing from ring of regen or Gaxx, I highly discourage from using them in melee. I will sometimes have mine dual wield when it's safe but in generally I keep them ranged. You actually want them to stay at ranged to dish out constant damage. Melee damage hits slightly higher to make up for the fact that melee attacks can miss a fleeing enemy which is greatly under estimated. Will murder undead, especially when buffed with greater malisson.
Mazzy - Best ranged NPC in the game. Most people hated her short bows but they don't really understand her or short bows (best bows). She can eventually use short swords in melee although she usually only has 1 star in them to start and does best to pick a better weapon like long swords(undead slaying+variety), flails(slow), axes(instant kill, azuredge), or katanas(stun).


Healer
On lower difficulties clerics can attempt to out perform druids. Skeletons are always powerful but cloud kill is nuts until Legacy of Bhaal. Sadly mages can also summon skeletons further reducing the strength of a cleric. It's still useful, it's just not what you'd call op which can cause clerics to drop off. Avoid Aerie. She has no hp and tons of spells but not a lot of high level spells.
Cernd - Hands down my favorite. Jaheira takes forever to max out levels and is always lack luster on any LoB play through. Druids exist for their quick leveling and quick spell gaining. Cernd is mostly a pure caster who can often go werewolf when there is like nothing to do. Druids have an amazing kit and any Player char druid is an amazing replacement to Cernd. None of the kits are bad and have a massive access to max level druid spells is just fantastic. They counter casters, do insane aoe damage with insect swarms, and elementals are crazy strong. On LoB upgraded elementals (random chance) dominate a lot of harder monsters and bosses. There is no cap to the strength of druids and Cernd is a hidden gem. At max level they gain elemental summoning and summon prince, both of which are super strong, neither of which cleric gets.
Anomen - Healthy hp, innate resists from shield, immune to missiles, can use a shield to reflect missiles, and has a strong selection of important spells like skeletons, and paralyze or stun dispel (only clerics get). The down side is higher level cleric spells lag behind. Anomen can use flail of ages (2 stars by lvl 12, worthless before) and mace of disruption +1/+2. You do not use him often in melee on higher difficulties because he loves to fail vs spells like fear, stun, or hold, forcing resets. High level cleric spells suck and LoB gives everything high saves.
Viconia - Low hp, terrible thaco and only 1 attack per round. Decent spell selection, can charm vampires (cheese with wish spell), and innate magic resist like Haer'dalis (65%+). Her low hp makes her a liability though. She can wear shields like Anomen to reflect missiles.
Jaheira - I never manage to leave her in melee. I prefer her to use two-handed weapons to keep her distance and stay safe. On LoB her spell selection lags behind badly and her melee and ranged damage output is pretty pathetic comparatively. Her thaco sucks until super late game when she starts to lap Anomen but she requires 6 million exp to unlock lvl 7 spells (iirc). Her kit is decent on paper but there are better options. If you play core, she's amazing.

Thief
Player char Gnome cleric/thief dual - You can't backstab but you can dispel illusions, use trap cheese, buff another fighter with extra thaco, summon skeletons, and combine two shit slots into 1 opening up ALOT of possibilities. The single greatest thing about this combo is opening up 3 warrior slots and 2 mage slots which is the perfect dream scenario for LoB.
Yoshimo - His kit is amazing. Shame is shame though.
Player char (Bounty hunter) - The kit is amazing and if you learn the line of sight for enemies where they can't see your thief even though they're visible to you and probably most of your party, you can cast enhanced traps from off screen and still land and trigger them instantly causing standard affects. Basically mid combat you go more than 30 yards and place BH specific trap and it triggers instantly on everything in an aoe. Broken and amazing. It's like the only reason to use a thief.
Jan - Brings backstab, illusion dispels, and extra magic casting. He can use Simulacrum helm and scrolls of breach, lower magic resist, and pierce magic to help shred boss magic resist and then start casting instant death spells like finger of death or chromatic orb to help one shot some bosses. He's actually very very powerful.
Nalia - A solid thief if you hate Jan. She gains higher spells but is designed to be a backup mage to Edwin or Player char.
Imoen - I hate adding her because you get her back so late. Her thief skills surpass Nalia but Nalia can use spells and potions on the few things she can't do. ToB is different where Imoen or Jan become much more useful.
Hexxat - I guess technically a thief. She's fun and flavor but not terribly good or useful. I only brought her with once for the flavor and the bag. Her traps do make her useful. Technically. She also much stronger on Core rule games.

Mage
Edwin - Stronger than even player char, he's pretty much a must.
Player char (Sorceress) - This kit is ridic. Dragon Disciple is eventually immune to fire and regenerates hp if you start at 18 con.
Jan - Doubles as thief. Can use simulacrum helm to utilize scroll and 2nd caster vs bosses.
Neera - Has 1 more spell per level than Nalia or Imoen. Her wild surges can be controlled if you avoid casting until you earn her robes. If you get her wild surge bonus roll high enough, you're fine. Most of the bad surges are at the very bottom of the list. She cannot party with Edwin. Ever. Completing her side quests unlocks a special shop with a slew of scrolls, wands, and wizard items ripe for pick pocketing in Amn. She also does well in lvl 3 of Watchers Keep in any red room because she has innate resist against wild surges. Her only downside is you really want to avoid casting spells with her until you get the chaos shield robes because there are some nasty surge rolls that involve losing all your gold and other pitfalls.
Nalia - -2 ac, -2 saves, fire resist ring. Can't use a 2nd ring for bonus spells. Gains spells faster than dual and multiclass. Simpler than dealing with Neera's wild surges.
Imoen - Like Nalia, lower level, stronger thief, chapter 5.