[Yossar]

Some of the best units in the game are blood units. I don't know why yours are dying. Maybe you aren't using enough? With devils and fiends of darkness, I find you need at the very least 5 or 6 to do anything useful, ideally more. Devils are some of the best units in the game and Fiends of Darkness are good until you can get Devils. Two of the toughest commanders in the game are ice devils and arch devils.

For pretenders, you can never go wrong with just making a tough pretender with any nation. A ghost king with at least air and water for quickness and mirror image/mistform is the "overpowered pretender of the moment". Or you can use your pretender to cast spells your nation normally can't. If you don't get mages with earth, take an earth pretender, etc.