Magic in your case is a power weapon, quite literally. It's common in video games to have such things, like a rocket launcher for instance. You may have only one shot with it, so when you pick one up, you make sure you use it wisely. That's just pragmatism, you only use has much force as necessary and conserve your firepower for when you'll actually need it.
Weapons tools made to kill other things, animals, humans or otherwise. And that are specifically designed to make this as easy as possible. By the way you describe it, it will always cost more energy to kill someone with a fireball than to kill someone with a weapon. So save energy, use guns*.
*Note: This is terrible advice out of context.
In a modern setting, it's guns all the way. You know you've heard once in your life that guns don't kill people, and really that's true, the gun isn't sentient, it's just a tool. But it does make killing other people bloody easy. You barely need any training. It's so easy a kid can do it and that's sadly true.
It doesn't mean magic doesn't have its uses. It's also a tool, but a different one. A very powerful, very costly tool. If you have to blow a hole in a wall in a hurry, it's useful. If you can use lightning to zap a vehicle's systems out, it's useful. It could be use to intimidate your enemy, or it could be used as a replacement for a grenade. There's a cost to using magic, just like there's a cost to using your rocket launcher ammo on that one guy. Sometimes it's worth it, possibly because you can't win otherwise. Sometimes it's not, and bullets are just a more effective solution.
In a lower tech setting, crossbows and bows are acceptable substitutes to firearms, although it would make magic would be comparatively even more powerful and less expensive. To the caster though, it will cost the same regardless, but the reload time on a crossbow is significantly higher than with an assault rifle, so it may be just slightly faster to throw magic rocks than it would be to reload a crossbow, aim and fire.
There's nothing a knight in heavy armor could do to stop a wizard from cooking them with a fireball, save for getting their sword inside said wizard fast enough. Generally speaking, I think it would be a good idea to stay in melee range. They can't use AoE magic against you (like fire, because convection), and if it takes time/focus to cast a spell, they can't really afford that cost when there's the imminent threat of a sword coming for their spine.
People in a medieval setting may also be very impressionable, so using magic might convince them to just run the eff away and not look back.
In your case of fighting against giant monsters, you'll probably need powerful weapons. Not knowing anything about those creatures, I'd throw everything I got. That includes magic, but that also includes tanks, trebuchets or other siege/artillery weapons. Always consider good ol' blunt force.