If this DM were somehow—I dunno—forced to have an enemy caster employ the spell spectral dragon against the PCs, and none of the PCs made the Spellcraft skill check to identify the spell as it was cast (DC 21),1 this DM would plop a Large blob on the battle mat and say something like, "The kooky dude in the pointy hat gestures extravagantly and speaks firmly, 'Ima makun uh dragon!' in Draconic and other syllables you don't recognize. Then in front of you appears a dragon-shaped mass composed of deepest shadow. What do you do?"
Which is probably part of the reason the shadowy form has an Armor Class. While the spell's description says the spell's effect "cannot be attacked" (which, yes: poor phrasing), I must assume the spell's author meant that the spell's effect can't be affected by attacks launched at it except for the small handful the spell lists. For example, this DM would allow the fighter to make attacks against the mass—the fighter's arm not disobeying the fighter nor his frightened sword refusing to enter the shadowy mass because the spell's description says the effect can't be attacked—, but this DM would then describe the fighter's sword blows as having no apparent effect. That is, this DM would let the spell's effect be hit easily (using AC 9)… yet to no avail.
Thus the spell's effect will keep on coming, biting for 1d6 points of Strength ability damage and 1 negative level against one foe per turn as long as the caster concentrates for up to 1 round/level.2 Sharp players should soon realize that the mass has no reach, makes no attacks of opportunity, has no threatened area and, because of how range works, doesn't move farther than 25 ft. +5 ft./2 caster levels from the caster despite the caster being able to take a move action to redirect the spell's effect to 60 ft. with the spell's range.3 I suspect especially smart players whose PCs have enough room to do so will simply tell allies to back up 50 ft. or so then have allies who can launch ranged attacks at the caster of the spectral dragon spell until the caster dies.
By the way, I suspect the real reason the spell's effect has an AC is so that it can be targeted by the 6th-level Sor/Wiz spell disintegrate [trans] (PH 222) and the rod of cancellation (DMG 234) (11,000 gp; 5 lbs.), both of which require their users to make a touch attack to hit the mass.
1 Such a failure is possible, I guess. A Wiz11 can be an overpowering encounter for some level 6 PCs (5% of them are; see DMG 49) and the two PCs with maximum ranks in the Spellcraft skill could both not get a 12 or higher on the Spellcraft check. Mean encounter, though. (Also note that this DM urges every PC to have at least 1 rank in the skill Spellcraft—it's a survival thing.)
2 Honestly, this isn't a very impressive effect for a level 11 caster's standard action. Such a caster had the 4th-level spell enervation [necro] (PH 224) four level ago, for example, and that spell only requires a touch attack rather than the spectral dragon spell effect's normal attack roll.
3 Another DM may give the spell's effect a reach of 5 ft. even though the spell itself doesn't instead of forcing the mass to move into a creature's space to make its bite attack. However, as printed, the spell's effect has no reach.
