The third plague lord is my favorite so far, because you have a lot more control over what's on your board compared to the others. The only thing I found dumb about the encounter is the insane amount of self-healing he runs. In particular, when he plays an Obsidian Statue in phase 2, the amount of healing he gets from it is massive.
I managed to take him down in one run (on Normal) but only because I had an OP deck and a Hearthstone to do it in two parts. I had the passive that makes your spells summon a minion from your deck, lots of huge minions (4x Kel'Thuzad), lots of cheap spells (6x Raven Idol, 2x Worthy Expedition), and also the passive that triplicates your discovers with a cost discount. My best-ever turn 1 (on an earlier boss, not the plague lord) was something like Worthy Expedition into triple Power of the Wild, which put a bunch of stuff on the board, including two Kel'Thuzads.
I'm currently thinking that Hearthstone is by far the best treasure, if you're trying to take out the plague lord in one run.