How does a 'Warlord' turn into a 'Duke of Ork'? How do I prevent it from turning into one?

I've been fighting against an enemy identified as a 'Warlord'. While fighting him, I noticed that he turns into a 'Duke of Ork'. He also appeared to deal and absorb more damage as a Duke of Ork:

This is in Ultimate Vault Hunter Mode (UVHM).

My questions:

• What types of enemies turn into a Duke of Ork? Also, do they revert back?
• How does a Warlord turn into a Duke of Ork?
• What changes when a Warlord turns into a Duke of Ork? (Any new attacks, behavior changes, etc?)
• Any tips or strategies on how to prevent a Warlord from turning into a Duke of Ork? In UVHM, my Psycho character can't seem to prevent it from leveling up even though I keep shooting it with my shotgun or ranged Buzz Axe Rampage.
• Any tips or strategies on fighting a Duke of Ork? (If I'm unable to prevent a Warlord from turning into a Duke of Ork)
  • Any particular weapons that seem to work better than others?
  • Any characters that are better than others at killing one?
  • Are they more susceptible to one kind of elemental damage more than another?

When a Warlord Grug/Turge levels up five times, it becomes a Duke of Ork. When you are not actively attacking them, they can do a sort of battle cry which levels them up. As you can see, the Duke is very high level and can dish out a considerable amount of damage with his giant axe/hammer. When you see a Warlord, take him down quickly!

There is a unique version called Warlord Slog. After leveling five times in the same manner, he will become the King of Orks.

_{Note: This Kotaku article mentions a leveling Badass Orc Warlord as well, but does not specifically say it can become a Duke of Ork as it does with Warlords Grug and Turge.}

The best way to kill the Duke after he has reached that stage is to remove all other Orks from the equation and then find a loop to run. Slag him with grenades or weapons and use Release the Beast and Buzz Axe Bombardier in unison to throw your axes as a badass psycho mutant while keeping him at a distance whilst you run that loop.

Alternatively, in the Mines of Avarice, have him plunge at you... off the edge of a cliff.