For those that are having trouble with pixes, equip Elwyen with Boots of Celerity, or anything to boost her Agility as much as possible, allowing her first action. With that action, activate Delorous Dirge, and keep it active as much as possible.
A wave timer at the bottom of the screen could be very useful for some of those super waves. Knowing when a wave is set to complete will add a good amount of strategy into when to use Rain of Fire