How about a Rare, an Epic, and a Legendary to choose from (they don't have to be new).
The challenge now lasts maybe 48hrs instead of 72hrs.
The challenge is to use your stamina to fight as many battles against this monster and collect cells depending on the number of survivors in each fight (like War Coin format).
This gives players a chance to fight monsters in order to win its cells. Over a period of 48hrs, even average players would surely collect enough cells to craft it.
I like the idea of each time you fight the monster, it will have different random immunities, so no fight will ever be the same.