After 20+ bg2 playthrough with many different mods, I can say I know the ins and outs of almost all of the game contents. I'm not as interest in the other rp challenges where if my main character dies (sometime I make a stupid mistake), I have to restart over in the beginning of the game. I think another tactical challenge is to force myself to play at a low level in the entire game. Is it possible to defeat irenicus, kangaxx or demogorgon w/ a lvl 7 party? I think this game is fun because you are forced to try new strategies due to having little to no access to higher level spells (limited number of scrolls for mage, no higher level spells for priest) and the party is very fragile throughout the game. Resource planning becomes even more important and certain npcs are more well equipped in some situations.
To play this challenge, the only rule is that you can never click on the level up button. You may max your levels in bg1, import to bg2 and abide to this challenge. I wouldnt start this challenge in bg1 because you simply dont have enough hp to survive (you will die from irenicus's animated torture before you start bg2!)
I think the easiest class is the barbarian, since you roll d12 per level and the barbarian rage gives so many immunities. For party, I think keldorn is a must for his dispel magic and true seeing. Viconia for highest level cleric, Jan/Imoen for thief skills, Edwin for mage spells and the last slot goes to either Korgan, Minsc, Mazzy or Valygar depending on the situation.