Challenge Rift (Daily)
The challenge rift is a daily event, allowing you to attempt 4 difficult bosses in order to attain pieces of a powerful equipment set. The bosses are all over level 100, and will be extremely difficult if you're not properly geared with potions. The goal is to activate every single tower in the rift in a single session, as leaving it or dying means you will be unable to access the rift until midnight GMT. The order the towers are presented below are the recommended order of battles.
Red Tower, Ifrit
The red tower summons Ifrit, Titan of Fire. He has fairly balanced stats and the lowest health of all the rift bosses, so he should be attempted first.
Blue Tower, Bahamut
The blue tower summons Bahamut, King of Dragons. He has the most defense out of any of the bosses at 52 fortitude, and a fairly high attack stat. He should either be attempted second or third in the run.
Green Tower, Odin
The green tower summons Odin, Undying Valor. He has the most attack power out of any of the bosses at a staggering 62 prowess. He also has the second highest HP, but almost no mana at all. As he can't heal much or use magic attacks often, playing defensively and countering his attacks makes him fairly simple to defeat.
Orange Tower, Shiva
The orange tower summons Shiva, the Creator. She is the final challenger in the rift, with the most health and mana out of any of them, so should be attempted last.
The Rift Portal
Once you have all four stones, you can return to the central portal and activate it to fight the final boss, The Rift Guardian. This is a superboss, and defeating it will complete the quest Secret of the Rift, rewarding you 500,000 coins for your efforts.