Really? Really? After all that happened you didn’t even bother testing the final task at least once? Are you freaking serious? How lazy do you have to be to not bother to at least test this one last task at least once? That’s beyond ridiculous! Unbelievable!
Edit: OK! I managed to complete task 10 by buffing him to 150+ so he would do 150 in a single attack. So the task is coded to check for 150 damage in one attack, not one turn. At least we don’t have to wait for them to fix it this time.
Edit 2: Well, since Blizzard’s new way of telling us what’s going on in the game is through some guy on twitter instead of the official forums, I should edit this in case someone sees this post before their answer. The actual error in the task is that it requires 200 damage in one turn instead of 150. The reason I got it by buffing him to 150+ was because it hit multiple enemies. So, it still counts total damage done in one turn, you just have to do 200 or more.