it's easy to kill the inferno because it constantly moves forward use first aid spell of your hero and take him to the end as a result the inferno follows the hero and is again at the starting
to kill undying primal there are 3 things 1)your units should have ring of strike (unlocked in extra) 2)you should have death because it has slash attack (the weakness of the beast) 3)the death should also have meteor lv. 2 (constantly keep attacking with it)
The hordes of enemy's units in higher levels are very dangerous so be careful . To take over them use frost giant, inferno, witches,centaurs,mermaid&epic tank