What exactly are the different targets like? How big/thick are they? What are they made of?
How quickly should Kohra expect herself to run out of mana? Not being familiar with the manga, I have no idea how much power Kohra can put out before running out of mana.
What are magic bullets?
1. Like a 1 foot wide, 1 foot long, 1 inch thick square, made of stone, with a glowing bullseye on 'em.
2. There's not like a hard and fast MP system; generally, when someone has spent a lot of mana it's equivalent to physical exercise. If you're "tapped out," you're breathing hard, sweating, feeling dizzy, all that kinda stuff. So just sort of think about how much "magical effort" you imagine Kohra expending per spell and describe things accordingly. I mean, Players are expected to be able to pass the exam, lol, so even if you're making it hard on her I don't think she'd pass out after the first events.
3. Magic Bullets are like your "default" spell, just a ball of magical energy that can be fired offensively. Like a generic ki blast in Dragon Ball Z, or like Magic Missile in DnD if it was a cantrip instead of a leveled spell. It can have qualities of the element or just be generic "mana." Again, depending on the amount of mana and the skill of the user, though, some people can "shoot harder" than others. I think there was one throwaway guy in the manga who didn't have a lot of flashy spells, but had mastered Magic Bullet well enough to use it like a sniper rifle.