For those of you who know or don't know, the Battle Arena prevents Magic from being used while having tougher versions of foes you seen outside to face.

BUT!

There is a trick to workaround with, but it involves bringing a lot of mind restores/Highs with you and the Uranus/Card of choice (Cockatrice or Thunderbird for heavy damage, Unicorn for heal (For Magician Mode as normal may not have it yet).

The best way to get these mentioned potions is to go to the Underground Gallery (Bring items that can increase Luck with the Venus/Mandragora DSS to boost it further) in one of the vertical rooms that endless spawns Specters, and start whacking them until they cough up one of the potions (I'm not entirely sure how much of the amount of the potions you can hold, but try to have 50+ MR and possibly 15+ MH).

Then if things get dicey in one of the rooms with tough enemies and very little room to move, just use a few and use the summon (unless you're EXTREMELY confident).