I think that for the final battle in BoBS you ought to eliminate the Dydd effect and allow the big foe there (trying to avoid spoilers

) to fight at full capacity from the word go - you will already have to make major changes to his spells (since the haste tactic just won't work now) so I'd recommend running through his spell list and making obvious changes
(including *put some clerical spells in there!*. Did Bruce Cordell just forget about that little detail?). At the very least he ought to have Death Ward as one of his 4th level spells and Heal as one of his 6th level spells. In fact I'd suggest eliminating circle of death and globe of invulnerability and replacing them with Heal and Contingency. Then give him contingency(heal)
Meteor swarm was a stupid spell choice to give him - it is only half as good as his natural breath weapon that he can use every 1d4 rounds anyway! The real obvious spell for him to have is Mordenkainens Disjunction, enabling his peerless body to take out any likely foe. Failing that Miracle is another obviously good spell for him to choose, as is Time Stop (to enable himself to buff himself up fully when he first hears a party)
If you have access to the draconomicon I'd suggest giving him one or two metabreath feats too. Clinging Breath or Maximise breath are good options.
If I was Ashardalon and I saw a pit fiend or something else immune to fire I think I might do one of the following:
either:
big breath weapon to clear out the lesser foes then attack the pit fiend
or
something to discommode the pit fiend quickly. Best bet for that might be the Destruction power he has, but that is only a DC26 Fort ST. Good but not great. Most sure tactic is Blasphemy since it will send any real pit fiends away (and may get rid of shapechanged ones too) as well as preventing anyone else in the party getting an action that round (dazed or better results against everyone).
Physically the crush attack is Ashardalons best friend as that can spoil the day for a large group of people, even the pit fiend.
I don't know if any of these ideas help. Losing the two spells per round of the 3e haste is quite a loss in terms of power, these suggestions might even things up a bit.