There are two immutable rules of magic :

1 : Magic has no limits.

2 : Magicians have limits.

Forget 1 and you'll be stuck in mediocrity, forget 2 and you'll erase yourself from existence. The line between these two truths is the path of true magic. The ability to do anything you can conceive. Or kill yourself trying.

When using true magic, none of the following rules matter. It's pure willpower in the moment and understanding of what you're trying to accomplish. A magician using true magic can do almost anything within human comprehension, and a little outside of it too.

Most magicians don't mess with it often. It's too dangerous and requires a high emotional state to access most of the time. Though all magicians who achieve immortality have used it once.

Ritualized magic : So how does less inherently dangerous magic work? Most spells are descended from 'life hacks' given to magicians by gods or other powerful creatures, or true magic that could be formalized into something less dangerous. Once you do something with magic the universe shrugs and says 'fine, that's possible.' But unlike true magic there are a lot more restrictions.

1) Base resistance : Reality doesn't like being broken. The more 'unreal' people find a spell the more difficult it becomes.

This is both the weakest and most random restriction. In high magic realms, people will just nod and say 'Okay. That's magic so sure." Similarly in any realm where magic exists there's always that sneaking suspicion that maybe a magician could do something so utterly insane.

Magicians frequently 'explain' their spells, or use tricks to make things seem more 'rational' to reduce this resistance. Mostly just to make minor tasks easier.

2) Personal resistance : For some reason people attempt to resist being set on fire or turned into a pig. The more damaging a spell is the more humans try to resist, calling up willpower they didn't realize they had to survive. This is why it's much easier to throw a ball of fire at someone then set them aflame with a nasty look.

Ritual connections, like blood or hair clippings, allow magicians to weaken this resistance. Getting someone to accept enspelled food or hitting them with a spell as a way of 'counting coup' can nearly eliminate it.

3) Understanding : The less you understand what you're doing and how you're doing it, the harder it is to pull off. Obviously if you have no clue what you're doing you're totally screwed. But even something as simple as creating a wooden ball can be messy if you don't know what makes different types of wood unique, and aren't sure how to structure the grain.

Study obviously helps with understanding what you want to do. In addition a lot of mages use association to help focus their minds on desired effects. For example using medicinal herbs in a spell allows a magician to understand the idea of healing better. But in the end there are always gaps in understanding that have to be papered over with magical power.

This point tends to be the biggest factor of a spell's 'difficulty.' A lot of the difference between magical schools comes from how well each school's ideology explains different facets of magic. Most spells are built to focus on understanding.

So how do you deal with restrictions other than working around them? Well just as there's three restrictions, there's three ways of getting past them. The latter two are major factors in magic on a day to day basis.

1) Power : Just as people have physical strength, they also have magical strength. Most people don't have much, but those related to supernatural creatures tend to have a good chunk. Power is great when you're learning, because you can brute force a lot of weaker spells. But it tapers off very quickly. And the natural advantages people like Helena have from being related to gods get washed away quickly by the general boost becoming immortal gives.

Power can be trained just like strength, but in general the benefits are so tiny it is better for a high ranking magician to practice anything else. Its main benefit is that powerful mages like Helena can push the limits of their spellcasting skills longer than weaker ones like Lyudmila. Something easily compensated for with more efficient casting.

It's also good in beam struggles, but that's asking to get blindsided.

2) Time : The more time a magician spends the better a result they can achieve. Part of it is the ability to gather more power and the sacrifice of the time itself. But the results are bigger than those factors alone would account for. Many magicians speculate spending more time acclimates reality to the spell's results.

Simply put the longer a magician spends working on a spell the stronger it gets, and the more likely it is to succeed. Time can also be 'stored up' by crafting items or ritual circles, though it's a little less efficient.

Pouring time into a spell is also risky however. The longer you spend on something the more chances you have to make a mistake. Spend three hours chanting and you'll likely screw up a word. Spend a sleepless night scrawling runes and chances are you botch something. And of course if you try charging up in a fight DBZ style someone will probably stab you.

3) Sacrifice : You've got to spend to gain. The quickest way to build up your spells is to use expensive ingredients, offering blood, or otherwise sacrificing something to gain an edge. The more valuable the sacrifice the bigger the boost. Throwing money at research is a common tactic for people trying to gain better understanding of certain spells. You learn the expensive way first and then refine.

Still there are strong limitations here. You can't just throw money at a spell (unless it's a spell to bribe someone). Sacrifice needs to be focused by ritual towards the spell's goals. Randomly injecting 100 dollars worth of gold dust into someone's veins to cure a disease will usually be less effective than giving them 5 cents worth of ginseng. And while ritual human sacrifice will get a lot of power, it does limit the kind of spells that will work alongside it.