It is impossible to solve the 6/8 Onyxia puzzle from the Book of Mercenaries, as one of the cards’ mana cost has been increased in a recent patch, rendering the puzzle unsolvable. Please add a way of refreshing our mana crystals mid turn, so we can finish the puzzle and move on to the rest of the content.
This is wrong.
The puzzle has been fixed and it is possible now.
It’s just that you didn’t find the solution OR you didn’t get the update.
You sure of that ?
The patch note only talked about the 2 boomsday puzzles that were broken, but didn’t mention the mercenary puzzle being repaired too
Edit : checked by myself that it has just been omitted in the patch
I am very sure. 
See here for exemple (to the end).