Probably not news to most people, but here goes:
Remember the room with 4x99 berserkers from BT1? There's an even easier and much faster way to gain ridiculous amounts of experience in BT2.
Go to level 1 of the Maze of Dread and look for monsters called Dream Mages. They do nothing but cast illusion spells (wind dragon, etc.). For best results, try to find a group of 3. Kill off all the other monsters in the fight, but leave the Dream Mages alive. Cast Disrupt Illusion (DIIL). Now have everyone in your party defend for a hundred rounds or so. The mages will summon illusions every round and your party will disbelieve them (effectively killing up to 3 monsters a round without doing anything). When you finally get tired of that and waste the mages, you can easily get 1,000,000 XP or so.
This also works to varying degrees with other monsters. Here are some others to try:
Sorcerors (Dargoth's, lvl 1)
Miracle Mages (Dargoth's, lvl 2)
Makers (Dargoth's, lvl 4)
Brockters (Oscon's, lvl 1)
In my experience, Dream Mages seem to be the easiest to find.
Note that this was based on the DOS version of the game. The same thing is possible in the Apple 2 version (the monsters sometimes behave differently in different versions, but I know it at least works with the Brockters) but the Apple 2 has a limit of 65,280 XP per character per fight.