The Dragon has an enormous amount of hp, so poison is a good way to bring him down. FB instead of Elhain. Especially if you use Valk with her counterattack.

Other obvious imrovement: bring Mausoleum Mage to lvl 60.

- Gorgorab 

- Warmaiden - Stun Set User

- Big Un

- M.Mage - Shield Set User

- Frozen Banshee - Lifesteal Set User

Raglin is another great adition if you see you're losing champs on the waves. One of the best legendaries in the game for me

I have not thought much about Dragon Dungeon tactic because it's kind of plain tactic, brute force, hit hard and he dies. He will raise his head 3 times, all you need to do is hit him really hard in all 3 times, don't let him complete the action. 

I would choose Gorgorab, MM + 3 attack champs that can do enough damage to lower his head. If 3 champs are not strong enough, then MM + 4 attack champs.