Brother MacLaren
Explorer
Derren said:3. Call lightning (storm) has only medium range. One move from the dragon and he is out of range.
4. The summons only last one minute. Not enough time to precast them and then approach the dragon. You have to cast them near the dragon (arrowhawks are slow) which makes you a target for the dragon.
Call Lightning Storm is long range (400'+40'/level, or 800'). If you get two bolts to hit (or one with a failed save), you have already done more damage than the dragon will heal that day. And if the dragon hasn't actually seen where the caster is (cover, concealment, distance, Hide, decoys, diversions, magic), he might flee in the wrong direction. Doesn't have to be actually stormy to get the 5d10 remember, clouds and wind are sufficient. From what I remember, a druid in a climate like Ireland or Scotland is going to be calling the big bolts almost every day. Summons last ten rounds, true; flying at a run it's possible to cover a lot of ground quickly.
The best bet is probably baleful polymorph, and that 9% chance of beating the SR and the save is *easily* acceptable as a plausibly backstory for how a 10th-level druid beat an adult black dragon single-handedly, especially because the druid could probably survive long enough to attempt it multiple times. Inflicting damage over time with hit-and-run tactics isn't bad either as long as you can pull it off consistently. As to the dragon spotting the druid, I would think that wildshaping into a small viper (or other animal with racial Hide bonus) and using Reduce Animal on self to become Tiny would be wise.
If you were considering non-core material, there's a spell from Draconomicon that either kills a dragon's flight ability (on a failed save) or reduces flying speed by 10' per caster level (on a successful save). It's not that high-level either. Such a spell would provide a major advantage to the druid, and this dragon can't cast dispel magic.