I found a secret level inside Titan Workshop. Basically it warps me to a challenge level, but only starring the autobots that I’ve already discovered in the main campaign so far. My objective is to kill 40 auto-bots, but the best I’ve gotten is 21. The nice thing is that if I decide to quit here, you get a skill point when you reach 20 kills. But I keep loading my saved game and trying it again. It’s very addictive! I can see that challenge mode is going to be digital crack for me.