tier 5 is a good start. we got a new millionaire center challenger, Insight, who may be appropriate for a tier 5 pet. I have this guide up with approximations of which challengers are for what tier. If you keep up battle quests and training, you will eventually be able to take on the sphinx.
the thing is that challengers like this are permanent. someone who cannot defeat the challenger now can eventually do it if they keep up training. if the challenger isn't difficult at all, they might as well put the loot prizes into the MC and forget about having a challenger.
Yeah I have a few permanent challengers I have been waiting to fight for example the torroweep that I got half way through. I need to throw some more money into weapons too. I tried Insight yesterday I am just short of beating it, I die and it has like maybe 50-100Hp left, so close I can almost taste it.
I am a little frustrated as well because I totally thought we were supposed to pick up the scrolls. That was the consensus from everyone when the first one came out I think. I only bought two of them but that's still 10 mil I could've saved and used elsewhere.