This may seem counter-intuitive to winning, but some of the challenges tend to require a great deal of luck in order to obtain crystal rewards. I'm currently bashing my head against "Win the fight with 9 or less HP remaining" against a Hangarback Walker. Annoyingly, I had this challenge in the bag by accident the first time, and then a runaway series of combos managed to trigger a healing spell, and, well...
If the CPU gets a good hand and favorable board layouts, it tends to snowball. If it doesn't, it tends to flail helplessly. So setting up a situation where I can still win with low health is iffy.
In my last attempt I managed to get it into a good spot, but the CPU needed one more turn to bring me under 9 - a turn it would not have because there did not seem to be any way to prevent my creatures from attacking and defeating the enemy (who was at 1 HP).
So is there any way to prevent my side attacking?