By unlocking and switching to the Dragonguard class 
for which you have to complete all the quests in Dragon’s Claw.
People love Gorgotha for its tanking ability, due to its third trait (75% skull damage reduction), cleansing ability (in case it gets entangled/stunned/frozen, etc.) and mana generation, from exploding gems. Filling up its mana can take a little while at the start, which would be one of its downsides, I guess, but there are ways around it (empowered troops, or patience 
), and once it’s filled up, it’s fairly easy to keep going.
Ishbaala is super strong 
and The Dragon Soul is pro at early (and late, I guess) soul-farming, as well as damage and mana gen/looping, especially when paired with a troop that creates Darkstorms/Firestorms (Umberwolf, Dragonguard, Sunspear, several mythics).
Oops, and exploding gems gives you 50% of the mana from the gems that get destroyed (but it rounds down 
or uses Banker’s Rounding, I can’t remember), and deals 100% of damage from skulls to the enemy. There should be a tool-tip 
.
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