In the game there is a cycle of "daily quests" that do not involve Doom Tower.
We are talking about the "advanced quest" here. If you can't beat the first boss in DT normal, you're not advanced. Go and lvl up some champs. Bring a team of 5 lvl 60 champs (and don't use HP burn) and you will be able to do the Magma Dragon on auto easily. You don't need any top tier legendaries to do that. A team just of rares can beat him.
Just to demonstrate I tried it right now with the following team: Apothecary (lvl 60, heal + spd), Athel (lvl 60, damage + weaken), Bellower (lvl 60, in a stun set, not even good at the dragon himself, just to do the waves), Reliquary Tender (lvl 50! Cleanse and revive) and Warmaiden (another lvl 50! Decrease def debuff). Just worked fine. Rares only, and two of them only lvl 50.
The mission is fair enough for an "advanced quest". There is absolutely nothing to be fixed here.
/edit: @ trevor.wilson
If you do a boss for the first time a golden key will be used. If you fight a boss you allready defeated before, you will use silver keys.