Can't complete Levequest 'Under Foot'

I'm having trouble completing the Levequest 'Under Foot' which you need to complete to unlock the Levequests for Costa De Sol. Each time I do it 1 monster doesn't appear on screen. It is there as I am losing health but it doesn't show on screen so I can't target it. It's very frustrating because I want to complete

All sorted I think. I was doing +4 levels but dropped down to +2 and now all enemies appeared.