It would be awesome if you could tell the maid / engineer where to go. I mean, don't send 4 maids to the same place when there is another spot that needs cleaning.
Found that when you attack a creature right at the edge of the screen that they sometimes turn invisible until you attack again. You can die like this if you idle and thought you killed the 'giant spider'.
This is why you get the 'Maximum Damage' boost when the next arena starts. It give you a bonus 15 seconds of all of your unlocked Active Abilities