The key to defeating Kuldath the Magma Dragon is the Provoke skill, as this prevents him from doing his strongest attacks and applying Hex to your team. As such, Kuldath is relatively easy (at least compared to other Doom Tower bosses) assuming you have Champions who can reliably do Provoke.
Basic strategy:
What’s important is to make sure you consistently have Provoke on Kuldath, which means either having several Champions that will cast it on him, and supplementing that with cooldown reduction and increase SPD / increase Turn Meter effects to ensure you can loop around and do your Provoke again. Additionally, you need to make sure to keep your Provoke Champions alive, so make sure they have a lot of DEF and HP and can get healing. Increase DEF and Strengthen will also help you keep them alive. Finally, Decrease ATK is also helpful to reduce the damage you receive from Kuldath.
Team compositions:
Here’s the team composition that I use:
Iron Brago (lead): Provides Increase DEF to keep provokers alive, great leader aura to increase DEF for everyone (as well as his passive), and a chance of setting a Provoke.
Arbiter: Turn meter increase, revive, and healing.
Apothecary: Turn meter increase, Increase SPD, and healing
An more accessible team (no Legendary and no Void Epic/Legendary Champions) would be:
Rock Breaker: Reliable Provoke with a short cooldown (3 turns)
Gnarlhorn: Reliable Provoke with a short cooldown (3 turns)
Seeker: Turn meter increase and small chance of Provoke
Apothecary: Turn meter increase, Increase SPD, and healing
For the 5th Champion you would probably pick another healer (e.g. Rector Drath, Jareg, Vrask). Ideally you would also pick someone with an Aura in all battles or Doom Tower to further buff your team. You could also bring in someone with Increase DEF or Decrease ATK to help with survivability.
Other notes:
Make sure you avoid Champions that do HP Burn as this buffs up Kuldath.
Remember that you can always use Taunting gear in order to give your Champions the ability to Provoke, in the event that you need a bit more of that in your team comp.
KULDATH THE MAGMA DRAGON SKILLS
A1: Brand with Weakness [ATK]. Attacks 1 enemy. Decrease Turn Meter by 50%. Will target enemies under [Hex] debuff.
A2: Overburn [ATK]. Attacks all enemies. Instantly activates [HP Burn] debuff on each target and decreases the duration of those [HP Burn] debuff by 1 turn.
A3: Stoke the Flames [ATK]. Attacks all enemies. Places a [HP Burn] debuff for 3 turns, then steals 10% of Turn Meter from each enemy under [HP Burn] debuff. Also places a [Hex] debuff for 4 turns on the enemy with the lowest HP. This debuff cannot be resisted or blocked.
Passive 1: Lethal Lava. 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.
