Reuploaded, now with 100% less resampling!
"I am randomly getting thrown off the ledge."
This is usually from an ability of the Lich King called Shadow Mine. He'll spawn a black circle around a player which will activate after a minor delay.
One of the ways to counter this is by running in big circles, that way you'll run away from any circle he places below you and you'll see all other circles coming your way.
Another way is to just stand still, where you can clearly see the floor below you. If he pops a mine on you, move back a few meters.
A third, more common variant is just tanking between the boss and his throne. This means if you were to get pushed back, you'll land comfortably in his throne.
"I got him to 30+ stacks but no achievement."
This is partially my fault, I never explained it correctly in the video. There needs to be ONE stack of thirty Necrotic Plagues, his buff only shows the sum of ALL plagues. His buff can be a good guildline but it all depends on how many Plagues are running wild.
To solve this, just tank him for a longer period, either try and find a single stack of Plague by going through all victims or chance on his buff.
"All ads die before they reach 30 stacks."
Make sure there are enough ads before you start stacking. Either decurse the first Plague or wait with summoning your companion.