That's because Gandalf's role in the story is that of a guide. he was sent down by the Valar as a counterweight to Sauron's rising power, but was not send to directly interfere. He was a sort of teacher so the people's of Middle-Earth could learn to fight their own fights. In the books he doesn't even partake in the assault on Mordor, but watches from a distance.
Gandalf also doesn't "resurrect" himself, he is, after death, send down by the Valar again because his job wasn't yet done and could be of more value as Gandalf the White, stepping into the role Saruman shold have fulfilled had he not be seduced by power.
One thing you cannot say though is that the world and systems of Middle-Earth are vague. Tolkien was anything but. The man was extremely meticulous in how he designed his world, it was literally an academic exercise for him.
I'm not aware of thenlimits of your magic system, so whatever I come up with isn't necessarily going to fit within your world building. But sure, I could figure something out were I to design a system. make it a very elaborate ritual, needing perhaps many people to focus at the same time. Or have it take a very heavy toll, maybe even needing a blood sacrifice or some sort of taboo practice the magic law explicitly forbids. In Fullmetal Alchemist for example, one of the taboos is to never perform alchemy on people, but one of the major plot points is centered around Philosopher stones, which is made from transmuting human souls.
You can go as wild as you want to with your rules, because you are the one who makes them.