Yes, it's about damage rather than  sustain. Think of it as dps check, to put this more of a mmo term. 

His Inhale ability (think it is called Scourge or Scorch or something) the one that turns part of his hp in purple, if not overcome can deal good amount of damage, ignore your team defence and can stun them. One of his other ability can place Atk debuffs on your team to further lower your damage. Almost seems like he doesn't like getting hurt :P

You MM probably died because of his Ally Protection - he takes half of the damage that Dragon dealt to others and that can be enough for him to die, especially after that Inhale skill. 

Stack poisons, Decrease Defense, Weaken, skills that geal gamage, based on enemy max hp and speed works better here. Block debuff/clean debuffs skills can help you too. 

Thank you for the replies.  Yes I was trying a slow and steady approach, but eventually he just one-shot my whole team from full HP.  Thanks I didn't realize he did so much damage to 1-shot MM via Ally Protect.  I have him with 73k HP and 2700 DEF.  He's pure tank, but got destroyed.

I have no problem beating Dragon 20 with 4 champions, but I'm trying to get it down to 3 (so I can use the other slots to farm food champions while getting artifacts). Anyone find a good combination of 3 that works?