Sorry if I sound cynical here but...what "casual" player would enjoy this? Casual players are casual for a number of reasons, but the biggest one is time commitment. Casual players don't have the time to practice adequately for an EX Primal boss rush that has be repeated in its entirety should one person mess it up. Heck, many casual players might not have time to do the entire boss rush. Keep in mind there are currently 10 Extreme Primals currently. To just run through all 10 bosses it will take a good amount of time. Many "casual" players struggle with doing one EX primal, let alone a ton of them. This is clearly hardcore content. That's not a terrible thing necessarily, but understand it won't have casual appeal.
That said, a "Boss Rush" mode could be really fun. I think a "Primal Rush" has a ton of merit, especially if you had a "Normal" mode for casuals and an "Extreme" for more skilled players. Give it some excuse like "The Wandering Minstrel is trying to tell the stories really quickly!" and put it in a boss rush. Stick in a unique mount for Extreme and Normal versions, and you'd have pretty solid, fun content that some might enjoy. I would personally remove the "Wipe and you have to re-do everything from the beginning" part. I don't really see what it adds to the experience, aside from extreme frustration when Bob messes up mechanic X and screws over everyone. I'd say just let them try as many times as they like, and just make the reward more for bragging rights then useful. The rewards:effort ratio is going to be really, really tricky with this sort of content.
