Some encounters are really hard just to beat, almost impossible to get achievement crystal too.
What is worse than jace pre-bounce nerf? An enemy with skill “6 loyalty return creature to hand. If hand is full destroy it instead”. 
I managed to kill it with jace setting everything for thooters and even with good hand he just kept bouncing tokens almost every turn 
That’s good though, the challenge is fun 
(Unless you don’t have the key cards needed to win, then it’s frustrating.)
They’re pretty challenging at times, which I like. It’s nice to have to think of how to beat them instead of putting one deck on autopilot and clearing them all in an hour.
One I found that was tough the opponent whose power was to give +4/+0 to all his creatures for like 6 loyalty. They got huge FAST.
Good newwwws everyone!
Defeating the last encounter(that is almost insane) gives a BFZ booster 
Absolutely, it’s good to have to consider how to build a specialist deck to beat some of these challenges. They’re meant to be challenges, after all. 
What annoys me is the speed challenges, “beat this in 8 turns or less”. AFAICT you really need to get exceptionally lucky for those - good hand plus good cascades plus the enemy not getting good hand or good cascades. 
Thankfully there aren’t that many of them.
The colour challenge is kindof nice too. Once I’ve worked through the main campaign I imagine I’ll spend a while coming up with the fastest / most reliable quick win decks for each colour, and go through each campaign level using each colour in turn.