You should NOT bring Sicia to the Magma Dragon boss as she would heal him with her HP burn.

Lethal Lava [Passive] 

Instantly places a [Hex] debuff for 4 turns on an enemy whenever they remove [Hex] debuff from themselves or an ally. Will ignore [Block Damage], [Shield] and [Unkillable] buff and will ignore 100% of DEF when attacking enemies under [Hex] debuff. Instantly removes any [HP Burn] debuff placed on Kuldath the Magma Dragon and replaces them with a 15% [Continuous Heal] for 2 turns.

You don't need top tier champs to beat the Magma Dragon. Like I allready wrote in another thread about this quest: "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."

 Fu-Shan, Thenasil, Kael, Dhukk and Coldheart should definetly be able to beat this dragon.

Thanks  for  the  feedback.    I  am  struggling  with  level  8.    Aoe  packs  are  wiping  me.    I  am  sure  it  is  a  combination  of  champion  turn  order,  gear  and  composition ,  but  i  can  not  clear  8  with  sicia,  coldheart,  kael,  50  fu  shan  and  Dhukk...have  them  all  running  lifesteal  plus  one  other  set..