In order to face the colossus, you must dispatch a total of eight Astromons; however, you cannot dispatch duplicates of the same Astromon, so be sure to take extra care determining which combinations of Astromons to use.
During battle, you will also need to strategically choose when to switch out each party. If you are able to defeat the colossus, you will be rewarded with exclusive, rare gems. As of the August 2018 update, the Colossus Dungeon has been reworked. Each entry costs 25 energy, with up to 2 victories per day. Further entry after 2 victories cost (n+1)*100 astrogems, n being the number of victories after 2.
Clear this dungeon 8 times each week to receive a random Skill Book.