Sorry, Littiz, I got my numbers mixed up. It has been a couple of months since I played Throne of Bhaal, and I was thinking of two different games. When I played all the way through Shadows of Amn and Throne of Bhaal with only two characters, they each had about 18.5 million experience points. I don't have a saved game handy to check the actual figures. When I took Sarevok with me, the numbers dropped quite a bit. My Cleric/Ranger and Imoen had about 7 million points apiece when they started Throne of Bhaal, end they finished the game without about 13 million points apiece. Sarevok must have had about 8.5 million. I did cheat in a couple of respects. First of all, I gave Imoen and my Cleric/Ranger an extra 500,000 experience points apiece when they started Shadows of Amn to bring them up the to level they were when they finished BG1. I also exploited my knowledge of the game to maximize experience gains.
Excluding Watcher's Keep, the total amount of experience your party can earn in Shadows of Amn is about 18 million. That's assuming that you undertake every quest, kill every monster, have your thief pick every lock and disarm every trap, and have your Mage memorize every spell. The actual number of points will vary depending on how many members you have in your party, since some quests award "quest experience points"--that is, a set amount of experience for each party member, rather than one lump sum to be divided among your entire group. Also, the number of monsters will vary depending on what level your characters are when you enter an area. Small parties which advance in level rapidly encounter more monsters than large parties which advance more slowly. There are also some areas where monsters respawn, and I made stops there occasionally on my way to other areas.
Including Watcher's Keep, Throne of Bhaal adds about another 18 million experience points to the game. The same conditions stated above apply. I did try to maximize my characters' experience. For example, I completely obliterated the enemy armies. I waited until they stopped respawning before I finished the area. My characters were over 20th Level (Imoen was about 30th Level) when they entered Watcher's Keep, and I'm sure that I saw more monsters than I did when I played through it with a larger party in Shadows of Amn.
I don't know why your party didn't earn as much experience as mine did. If you had the game set to "Normal" or "Easy" instead of "Core Rules" or higher (I'm not saying you did), that might have made a difference; perhaps the game awards less experience with those settings.
As for the final encounter, well, the Ascension mod makes some changes that I don't want to discuss here because they're spoilers. But I will say that I probably handled the encounter(s) the same way you did: trial and error, and lots of reloading.
I wouldn't say that I used every trick in the book (I'm not that good), but I did try everything that I could think of. And also...well, when I played the game with only two characters, I didn't actually finish the game, because the final encounter was too difficult for me. That's one reason why I brought along Sarevok the next time, and that's when I installed the Ascension mod.