[BUG] Mektul The Forgemaster can be cheesed 100% of the time with this method.

Method
1) Have enough stun buildup on your primary sources of damage to ensure that boss can be heavy stunned.
2) Get boss to approximately 80% hp before he walks back to his lava to do his special "walk with lava" move. Also make sure the boss is primed for a heavy stun and do not heavy stun him before.
3) Heavy stun the boss as he is walking back to his lava. I use staggering palm.
4) The boss is now bugged. He will walk toward you doing nothing for the rest of the fight. To properly exploit this run back and check for an essence so it doesn't get in the way. Freely kill him or use him as a dps dummy. Be careful of spikes nearby you on the ground. When the lava goes over them they will pop.

Bug Showcase
https://youtu.be/JxgKPI-Dgzc
This works 100% of the time.