Ok everyone, what do we know so far? There's a huge green dragon, that presumably has a poison breath weapon. Terrorizing these villages, killing Elves. There appears to be a rogue Elf, that learnt the arts of a "High" forest mage.
There is a dragon rider, possibly same person. The dragon can become invisible, and expell poison gas, and or fumes?
Personally I would rather not take them head on in the thick forest. I have visions from the movie "Predator," where it can go stealth, invisible, then pick us off one by one!
But like a Romulan "Bird of prey," would need to become visible to unleash its poison breath? One can only hope, then hit it with everything we got?
If only we could lure it out of its den?or out of the forest? It could even be watching us already, invisible? We would need a passive perception, greater than the creatures dexterity to know where it is. Unless it makes tracks, and noise?
At this rate, may have to go back and warn the other's back at the castle, prepare for battle stations, so to speak.
Or, use voice of the "Chain" send the psuedodragon back to warn Cratol, by speaking though it?