Lower Resistances does not last for 2 seconds.
[img]tongue.gif[/img] Your Mage was most probably of too low a level for the Shadow Dragon to notice a difference. Cast 2-3 of them instead.
However... your party is not very magic-oriented, so I see no point in lowering the SDragon's Magic Resistance in the first place anyway. Fill up all your level 5 spell slots with Breach instead.
Get Jaheira to summon some Mountain Bears, while Aerie summons some Aerial Servants. Position the summons in front of the SDragon, while your Fighter, Cavalier, Fighter/Thief and Yoshimo stay at the back of the SDragon. Aerie and Jaheira should standby on one of the SDragon's side, within ranged distance.
When you are ready, get Aerie to cast a Minor Spell Sequencer with Chant and Bless on the tanks followed with Haste. Once boosted, start chopping. Use Breach and Insect Plague on the SDragon once it turns hostile.