The easiest way to get the hard badge is to setup a 75 card deck aimed strictly at building. I beat the last three levels on the first try with this one:
Add 5 of each of these in your deck:
Row 1: nothing
Row 2: Guards
Row 3: Mages, Pixies
Row 4: Add Crystals, Add Bricks
Row 5: Everything besides School
Row 6: Fence, Wain, All Bricks
Row 7: None
Fill out the remaining slots with your favorite cards until you hit 75. (I'd recommend Reverse and Magic Defense.)
Just generally ignore what the computer is doing. Play a Fence early if you have it. Once you get your wall to 20 or so, ignore it. Prioritize adding more builders and mages unless you are less than 8 turns from winning.
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