It seems to me that the best strategy (barring Shivering Touch cheese) is to find a way to cool the lava around the dragon, trapping it (at least temporarily). Even if its head is free to attack, if you can at least trap the legs you'll limit its mobility and knock out a couple of its natural weapons. There are a couple of spells from Frostburn that might be useful for this:
Flash-Freeze is a 2nd-level druid spell that drains all heat from a volume of earth, stone, and/or water 10-ft square and 10-ft thick. This probably wouldn't trap the whole dragon, but you could at least bind up its front claws.
A potentially more effective option would be Control Temperature, which is a 3rd-level spell for druids, sorcerers, and wizards. This lowers the temperature in a very large area (emanation with radius 20 ft per level, centered on you) by one band per five caster levels. By RAW, this would drop the temperature to a maximum of 140°F if you cast it at your own current caster level, or 110°F if you got a CL 10 scroll and took your chances with the CL check (you'd have a 20% chance of failure). However, this spell might not be so effective depending on how your DM chooses to rule it -- does it affect the whole area, including the lava, or just the air temperature? With lava being so incredibly hot, would it still be lumped into the uppermost "extreme heat" temperature band (allowing you to bring it down to the temperatures quoted above), or would it be treated as a still higher band (in which case you'd only cool it by 30°F, or 60°F with the CL 10 scroll)? If the rules that the spell does affect lava and doesn't put the lava at a higher temperature band than "extreme heat", this would be devastating even at your own caster level, allowing you to solidify a very large expanse of lava. However, if your DM rules that it only affects the surrounding air or that the lava belongs in a higher temperature band, it would be pretty ineffectual.