I dunno about Lord Jierdan, but I was able to kill the shadow dragon in a toe to toe fight.
In fact I was able to solo the shadowdragon with my main character, a Kensai/Mage.
While that does sound ridiculous it can be done. One of the biggest keys, believe it or not, is one of the earliest magaical weapons you get, the Flail of Ages. This weapon casts a no save slow spell when struck. Combine that with the Robe of Vecna's casting speed bonus and Boots of Speed, the fight becomes amazingly fair. While I did have to toss some spells like breach and some quick magic missles (courtesy of a minor spell sequencer) at him, most of the fight was melee.
You also need a very low THAC0 to do this, I had to come back after chapter 3 to pull this off, because you can get the Amulet of Power from Aran Linvail (another key item). The amulet makes you immune to level drains, which is a must.
Equipment/Spells I was using:
Lower resist, lowers +20% resist, AND it's cumulative!
Tenser's Transformation, doubles hp and raises combat ability.
Minor or regular Spell Sequencer, this is important because while under affect of Tenser's Transformation, you can't cast spells. But you can still use spells that you've stored up in a seqencer.
Contingency, same reason as spell sequencer.
Ring of Kangaxx, combined with Boots of Speed, you regen about 1 hp per second.
Breach spell, if you don't have this spell, and if he casts Protection from Magic weapons (and he will) you've already lost.
Robe of Vecna, makes most spell casts near instantaneous with it's -4 casting speed bonus. Also sets base AC to 5, awesome for kensai/mages.
Amulet of Power, Immune to Level Drain and another -1 bonus to casting speed.
Flail of Ages, no save slow, this works great in almost any tense situation, not just dragons. Make sure you have some skill in this weapon, 2 slots is enough, can't slow what you can't hit.
Celestial Fury, my most skilled weapon proficiency, thus giving the lowest THAC0 available to me.
Ring of Protection +2, more ac never hurts.
Sewer Cloak, more AC.
While these tips are specifically catered for the kensai/mage. I've heard of people who have soloed the dragon with a Half-Orc fighter/thief.
NOTE: Robe of Vecna is only available in the collector's edition.
"Gentlemen! You can't fight in here! This is the war room!" -Dr. StrangeLove
"Never in my life could I have afforded to be raised!" -Fezhak
"I don't want to meet someone who shares my interests. I hate my interests." - Steve Buscemi Ghost World