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When you have the choice, put weak ghosts in the corner spaces. This allows the players the possibility of saving time and killing two ghosts at a time.
Use the Buddhas as much as possible. It takes two turns to set them up (with the double-action monk) but automatically kills two ghosts - even big ones that would normally take longer to kill.
Only engage with a ghost if you're sure to kill it or only need one die to come up to the appropriate color. If you rely on two dice coming up your way, you will likely be wasting an action that could have been used elsewhere.