Hey there!
So the easiest way to catch the intruder is to narrow the options twice and then guard a place on the third go around. You might or might not hit him, but each time you fail Petra will rule out one of the locations, so you'll get him eventually. (You might get lucky and nab him the first go around)
Another bit for Chapter 2. If you talk to Mason in Chapter 2, since you survived Lazren's initial test, he'll actually talk to you about his past. This will put you on the path for another soul gem (there's one extra in chapter 2, but Sara is genuinely hard to get since she requires the Casino)