[@Remram] You don't have to wait in "post order" to match your number or anything, just treat this like a sort of montage jumping between different characters. It's just, "How does your character perform each test? What's going through their head?" Etc. 

Spells work like most special powers in anime. People shout out things like "Wind Magic: Whirling Dervish!" or "Cyclone Spear!" and then destructive stuff happens. I would assume based on what's seen in series that spells require a minimum expenditure of mana in many cases (can't cast them if you get below that threshold) but that pouring more Mana in it makes it bigger, more destructive, faster, etc.