Build around the burning blade
Start with a close-range defense. The Malignus press kit describes the Cinder Demon swinging its burning blade from within a ring of fire as adventurers approach. Choose a place where the Demon can fight incoming adventurers without having to cross the entire hall to reach them.
Give your minions room to help
Keep minions close enough to join the fight as adventurers close in. If enemies are moving past the Demon toward the hoard, reposition the defense instead of following one target away from the rest of the crew. A beta player described choosing the Demon for a brute build, which fits this direct approach.
Keep the death blast separate from ordinary attacks
The press kit also describes a blast that spreads through the hall as the Cinder Demon dies. Plan your ordinary fights around keeping the Demon alive and using its burning blade and fire-ring setup. A death effect is a poor basis for a defense you need to use again.
FAQ
Is the Cinder Demon a good choice for melee?
Yes, if you want to fight adventurers up close. Its documented showcase puts the Demon in the hall swinging a burning blade as enemies approach.
Should I defend alone with the Demon?
Keep the crew involved. Minions give you another way to respond if several adventurers reach the hall at once.
Is the death blast a move I can use throughout the fight?
The press kit identifies it as a death blast. The burning blade and fire-ring setup are the examples to use for ordinary defense.