How do you get blue mammoth idols in dungeon blitz?

There is no such thing as a "blue mammoth idol" in Dungeon Blitz. It is likely you are mistaken or referring to a different game.

Dungeon Blitz features various idols with different abilities and rarities, but there is no blue mammoth idol mentioned in the game's official documentation or community discussions.

Please clarify which game you are referring to, or provide more details about the idol you are looking for.