Red Ring in Phantasy Star Portable 2 Infinity?

I was looking through an online guide and saw that the Red Ring is available as a shield,but a lot of the text was in Japanese.I'm trying to find out where in the game the Red Ring is...what missions do I need to do,what difficulty,do I need to be a certain character class?Any information would help.I have 2 characters that have beaten both Episodes and all side quests in the game so far,so I have access to many of the missions.

To obtain the Red Ring you need to clear stage 7 in challenge mode within the target time. And for that you need at least a second player to do it.

Rapid99