Ok I forget the name of the challenge, but it's located on basic campaign, in the middle top right after you beat jace. I know you have a lord of the pit and some other black vampires. I spent 40 minutes on this last night and could not solve it. frustrated I joined up with some friends in party chat and they have not been able to solve it either, so we worked together and have concluded that the puzzle is unsolvable with the cards on the board. Perhaps with old rules where you could sack after blocking it could be solved but not in its current form.
you have 1 life, no matter how you block you are going to take at least 1 damage from the goblin if you sac your cultist to gain life and 2 from the collosus that is 9/9 trample.