While playing through the puzzles in Book of Mercenaries - Mage Section - Onyxia of the void I hit a roadblock during puzzle 6 due to the recent nerf to the card “Snowfall Graveyard” which increased the mana cost to 5 (previously 3) and thus it is impossible to complete due to insufficient mana to execute the intended solution.
take a look at https://us.forums.blizzard.com/en/hearthstone/t/onyxia-of-the-void-impossible-post-card-nerf/12697015 see if that will work.
google the exact fight, puzzle or w/e, may take some time to read to the end of comments or comments after strats posted, but it can be done.