Is the "main character who we all expect to die after fatal wound lives thanks to magic" too cliche, or could someone pull it off? I've sort of planned it to happen in my urban fantasy...
Using magic to solve problems is bad storytelling. You could invent a million other reasons why the MC is still alive. Make up your mind and be creative.
Anything can work if you do it well. That aside and considering I have no knowledge of the scene, the context, the story etc, my personal preference would go against doing that since many variants can leave the reader feeling cheated, like the author took the cheap way out, especially since there are many other ways to pull off the same effect.
Magic itself is not inherently cliche, but I would be conscious of how you intend to use it, why you intend to use it, and the effects of using it. Keep in mind that many things come at a price. Be creative. Think of all the open possibilities. Just remember, it can't be too easy.
If magic is used for something as big as healing fatal wounds, someone needs to pay the price. For example, the principle of "backlash magic." The character is dying, and someone uses magic to heal them, but they, themselves, take on the full extent of the injuries they've just healed. But, as has been said, you need to consider all other possible answer before you settle on the use of magic. I honestly read in a book once where the MC is walking through the makeshift hospital after a battlefield, looking at all of the dead and dying soldiers that "just couldn't be saved." Said character, after being in a battle in which they received injuries that may have been fatal (the author wouldn't even go so far as to say they were fatal; only that they might be), was healed by means of magic. It was mind-boggling. About two chapters later, I put the book down. That was at the age of about 12 or 13.
I don't know if cliche is the word I'd use to describe it. I guess you could make the main character survive fatal wounds because of magic, but something's got to give. In Revenge of the Sith, Darth Sidious uses his force powers to save Anakin/Darth Vader from dying in the lava fields of Mustafar, but it's not without cost. Anakin's to live out the rest of his life in a robotic suit, with several of his limbs missing and horrible scars over his body. In that case, the consequences were not on the healer, nor did the healing itself really generate any consequences. As long as the character doesn't live happily-ever-after because of a magical force that saved them, I think it would be fine. Edit: It's important that your main character doesn't have an easy journey, struggle is what strengthens us. If your character is being saved (without consequence) by powers which can't be explained in real life, the struggle is going to seem absent. Minor characters can be saved by the supernatural without consequences, main characters can not. My opinion, of course.
I think it depends on how you execute it. The use of magic itself is not necessarily a bad idea, you should simply make sure that there is a reason it hasn't been used to such an extreme earlier in the story. This can be things such as the cost of the magic on the user, or something I personally prefer, the use of that variety of magic going against the characters personality. Either a breaking point or a revelation. For example, in The Legend of Eli Monpress, Josef Liechten refuses to use the greatest magic sword ever made not because of any cost associated, but because he wishes to win his fights as the greatest swordsman, rather than as a swordsman with the greatest sword. He resorts to using the sword against a superior opponent as he is about to die so that he can live, and continue his quest to become the greatest swordsman. So there was never a physical cost for the magic (which in my opinion is cliche), and in the end he betrays his own code to further his goal. I'm not certain that that makes any sense, but I hope it's a useful example.
As others have said, anything's possible if you do it right. I'm in the camp of "right" involving some sort of backlash or payment. If it's just a free pass, it could look suspiciously like you just weren't willing to let the character go, and then your character becomes yawnfully immortal. The Dresden Files by Jim Butcher used to be my favorite book series. Gradually it got out of hand, things got too grandiose and too over the top. Eventually, [SPOILERS] the main character dies. But, of course, the series wasn't even half over. There was no doubt he'd live somehow. (the series is first person, past tense) I more or less stopped reading at that point, as I knew the last semblance of what I loved of the series had died. I suppose I should read the book where everything's taken care of (I do own it), but I have little to no hope of it being satisfactory to me. Yet, Butcher is still a New York Times Bestseller and did get his books turned into one season of TV show on Syfy. So take from this what you will.
If you use magic to heal a character, there should be some lasting effect or trade off. As someone else previously mentioned, it needs to be believable, especially within the universe you create. If you just have magic come straight out of left field and BOOM! The MC is back at 100% like nothing happened, you're gonna end up with a lot of frustrated readers.
