Global Boss Challenge

One question: What preparations are we allowed to make before the battles? Obviously going out and levelling to 99 or learning spells would be illegal, but what about buying items (Did you give us GP for something)? On the other hand, we can clearly equip ourselves, but would we be allowed to cast buffs on our characters before the battle? Or possibly get our characters to low HP before fights?

Finally, are we allowed to switch characters, or do we have to use the ones in each video?

I feel like it's okay to make preparations as long as I don't gain any new items, win any encounters or change characters but I figure I should ask first to make sure.