When you get to challenge 300, pick someone cheap and lose fifteen times (keep it on, just keep pressing try again and go do something else in the mean time). The AI adjusts itself. After about fifteen times, it just lets you beat it.
I've been stuck on challenge 167 for DAYS. I was thinking about skipping it, but I want the trophy, so I'm just going to invite someone over who can pass it for me.
Problem solved. :]