Here’s the best way to gain the most EXP while getting the dragon summon at the same time (tested on DE):
1. Proceed through the quest until you get to Brahmos and the 4 dreamers. Make sure you spawn the Red Prince in the previous arena fight.
2. Obtain the key from Brahmos either by persuasion or pickpocketing. Leave one party member behind.
3. Have another party member (preferably with high persuasion) go through the door. Transform into Lizard Form using the Shapeshifter Mask and approach Sasha.
4. Pass her persuasion check and save her. You will then obtain the quest rewards and EXP. Once done talking, use Flame Breath on the egg (it was invisible to me on the pedestal but still worked).
5. The dragon king will spawn and a fight will start. Run away from the fight by sneaking and teleporting using waypoints. You will then have to travel all the way back to the Consulate and into the portal again.
6. Meet up with your other party member and kill Brahmos and the dreamers. He drops a unique blade.
7. Go through the door once more and kill Sasha and her goonies. Do not kill the dragonling.
8. Once everyone but the dragonling is dead, run away from the fight. When aggro is dropped, the dragonling will turn neutral and you can then adopt it. Make sure your character has high enough persuasion.
9. Go through the exit portal and quest will be complete.