In the Battle Arena, equip a good DSS Summon Combo (for example, Uranus + Cockatrice). Press Start, select "Items", then "Use", and use a "Mind High" (not a "Mind Restore" -- however they can work, but you will need to be extremely fast). Immediately after exiting the pause menu, press the DSS button. When your character is done casting, press Down, Forward, Up, Attack. If done fast enough, you should have pulled off a summon in the Battle Arena.