Personally, going by the Descision Making segment of the RtL rulebook, page 10, whenever I find that the monsters make sub optimal choices I try to take a few seconds to think like the overlord and choose for them.
 
That is to say I try to never let actions go to waste if I can help it. You're free to play it whatever way you want, but since the app does not keep track of map positions I find some times the monsters don't make the best choices. It is to be expected in opposition controlled by AI.
 
The players can scale difficulty in this way and I enjoy having the flexibility in the rules to do so.