Hello,
There is a known bug with this unit. The splash damage on too many targets can cause the game to freeze. The devs have been working on this and never versions of the client do address the problem and reduce the amount it happens, but unfortunately the problem is persistent. At this point in time, we can only recommend one of several workarounds:
- Use a different units
- Minimize the screen when the mystery goes off
- Use the boss key when the mystery goes off
- Use his alternate mystery
- Wait to mystery when fewer targets are present.

Sorry for the trouble here and hopefully you can manage to work around the issue.

No problem, best of luck, and I am genuinely sorry you are experiencing this. :(