some considerations before we start.
-even if i like to play with 3 or 4 people parties, i find it the sweet spot between soloing and full party, 3 is my perfect number as i like to micromanage, i recognize that the initial advantage of leveling faster in end game is lost, as in end game even in a 6 people party they are well developed and the number makes the difference.
-full parties level up very slow and some well known tactics, like learning and erasing scrolls, if are effective for dualing minimizing the dual time, are not effective for boosting the level of a large party, there are not enough scrolls. other sources of free experience, like abusing of exploits or turning to stone and back to flash foe to get multiple times the xp will not be considered in this topic, i am talking here only of making the best use of the xp normally available in the games, as i play bg2 in that game, but for the others it works somehow the same.
-not all the classes need the same amount of xp to be effective, multi are the ones that need it more, some other class will benefit of fast leveling, like cleric to make explode undeads or sorcerer to be able to cast high level spells when a regular mage don't have even access to the scrolls to learn them. plain fighter types imo need it less and duals, if done at the right level, tipically 7 or 9 in bg2 either.
-xp comes in 2 ways, for the kills, and less people is in the party the more each one gain from it, and for the quests, and here the more the better.
putting together those considerations the logic powergaming choice of a strategy imo is obvious, but as i did not read of it anywere, probably is not so.
the goals are
1- to maximize the number of people in the party when quest rewards are given
2- to make it the smallest possible when the xp from the kills has to be shared, having in the party the characters that need more xp to be well developed
3- to play the first part of the game with few people in the party, better alone, to speed up the leveling of charname, until he reaches the ideal level that allows to recruit npcs at their maximum possible level. this is not relevant for custom parties but makes an huge difference playing with npcs.
so the most effective way is to have a carname that needs experience, sorcerer or multi, also 3 classes type.
tell imoen to find her way out of chateau irenicus alone and solo the full dungeon, never going near minsk and jaheira to avoid triggering their dialogue. when the dungeon is cleared and charname is ready to go out of it he finds M and J, frees them from the cages so everybody can exit and gain the quest xp.
then he can drop them or keep them depending on his needs. jaheira can be dropped only once but being multi is one that benefits of getting more xp then others. same aerie if you romance her, but the romances with single classes and anomen should be avoided from this PG perspective.
then charname continues alone or maybe with an helper until the ideal level that allows to recruit at the maximum possible level is reached. at this point he recruits a full party.
from that moment to the end he drops and recruits them multiple times to have few of them sharing the kill xp and the whole party as some quest reward is given. this can be done in sinergy with the game, in a quest where many undeads are found, or even in a single phase of a quest, like a certain room in firkraag dungeon, he can have a cleric to turn and use negative plane protection, and so on. more helpers can be recruited for the boss battles if needed, while the other battles in between are less challenging so only the few that need more xp can be present. this mean for some quests and for the whole asylum-underdark to recruit everyone to go to the next area or get a quest reward and drop them as they are not needed. for other quests you can leave some party members at copper coronet or wherever you like them and optionally delay quest rewards just as you do for fast dualing.
as the xp for the quests is a good percentage of the total xp in the end even those that have participated less to the battles in the end will have some levels, while who needs more xp will grow at a the same rate of a 3 people party being in a 6 people one.
to be more precise we need to know the exact composition of a party, but as you get the principles you can easily adapt them to the particular situation.
let's make an example
1 charname fmt
2 aerie (charname' romance)
5 haer dalis
1 and 2 are always present and as multi they gain from it, charname cause has 3 classes to rise, the whiny one cause the earlier she can mix arcane and divine in triggers and CC the better it is. she is also the divine caster so to have her growing at the rate of a 3 people party make her turn undeads effectively and all the rest.
3 and 4 are fighters and somehow are interchangeable, you can take the pally when mage fights are there and mazzi when a good ranged fighter is useful, for some quests you can go without them as a fmt and to some extent aerie can hold the front line, even more being overleveled for the phase of the game. the blade if present in that situations help them.
5 it depends on the use you want from him, you can focus more on him and have him more present or regard him as the 6th of the group and give him little more than the quests xp (minsk, anomen and vaigar can cover the same role) and in tob maybe will be dropped to make place to sarewok.
6 is the main caster and as a wild mage needs some caution, as the more she levels the more she is able to cast without the risk of a sourge, probably she needs to level up faster in the beginning but as she begins to be very strong each level more is less game changing, let's say from the moment she has a couple of lev 9 spells. at that point you can have her less frequently in the party, as 1, 2 and 5 if present cover the arcane casting quite well.
in the end you have charname leveling at 2/3 of a single class and aerie at the rate of a single class, neera leveling slower than a single class but fast until she reaches a good level of power, a couple of fully operational fighter types (after a certain number of levels they reach the better thaco that is what really matters in that party that have plenty of arcane and divine power to substitute GWW with IH and gate better helpers than keldorn's deva. and haer dalis as 6th with a level enough high to be useful at almost 0 cost or a better level without nerfing too much the growth of the others as you prefer.
i know that the party of my example is not a perfect PG party, and i have chosen it so on purpose, the system works with every party. with a custom PG party works even better.
you will have a fmt and a cm that grow really fast so they reach good power level much earlier than having always 6 people in the party, a main caster that reaches good levels quite fast (don't forget that with the initial strategy every npc is recruited early, but at a relatively high level, and this more than compensates the fact that later she looses some of the kills xp). and you have 2 fully operational fighters, maybe under leveled than keeping them all the time in the party, but everybody benefit from the recruiting boost, and an other helper that can be useful as can tank, can help casting and somehow can also fight effectively. the overall efficiency in each phase of the game is superior to the one having everybody always present in the party and sharing all the kills xp.
you will level slower than in a 3 people party but for every difficult battle and in end game for all the battles you will have 6 peoples ready, so much more apr, 5 casters, the main one potentially the best caster of the game as she begins to use dwehomers (that casted from a clone imho are much less riskier as some bad happenings happen to the PI) and 5 able to be in the front line (or be a ranged hurricane in the case of mazzi) and be effective there. don't forget that haer dalis can tank like the betters while equipped with 2 +1apr weapons and aerie and her simulacrum shapeshifted into golem, werewolf or mind flyer, with clerical buffs on top can be more deadly than most of the fighters. she will be ready for that and for the other high level cm combos fast as a single class grows normally, not only for the few final battles of tob.
it is not like having a 6 people party leveling at the rate of a 3 people one, that would be impossible, but it gets really close.
suggestions and comments are welcome. and i hope that it is useful as i never read something similar so i doubt that other people than me uses a similar leveling strategy.
comments from a PG perspective, please, as i know that rp wise the whole thing stinks bad but this is a PG topic...