You need to get 5 hero strike runes, legendary, and you only get 2 from missions. The other 3 are luck based when you buy legendary rune packs. When you have 5 put them on each characters last column. Now you have infinite tries on an endless mission. The reason for this is that each character takes a different amount of time to cast the spell so it may take timing to get it down. I suggest pausing the game with

first.