After the nerf to Forsaken Lieutenant now costing 3 mana the puzzle does not work. It also seems to be a problem in the interaction between Snowfall Graveyard and Elwynn Boar.
I tried this solution but the Puzzle bugs out because even though eight boars (need seven) died i did not get the sword. Book of Mercenaries - Dawngrasp onyxia of the void puzzle - #14 by EvilSmoothie-21930
The problem is that the double deathrattle of the boar is not counted with Snowfall Graveyard. Only one of the deathrattles tick so I am stuck at four and not eight when following the instructions. The other deathrattles do work though, like the mechanical sheep explodes twice.