I found a really, really easy way to do this achievement. I just want to add to what The Trumpet88 is saying. Basically, make a neutered deck for the computer to use in a practice match. BUT if you leave some blue mana and let him use card draw spells, the computer will cast those spells, lowering its library count. ie if you let the computer use fleeting distraction and inspiration, all you need to do is press Y until you win.