According to the books, the simplest shield spell was Protego, which Hogwarts normally taught in fourth year. Even then, very few students could actually perform it. One of its requirements was a genuine fear of harm.
Once someone cast it successfully with that emotion, any later casts no longer needed the fear. It was an unusual requirement, and also a difficult one to satisfy inside the school's walls. Most people who learned it properly did so through sparring, using duels to simulate real combat and produce an actual fear of being hurt.
...
Julian, however, planned to be far crueler to himself. He intended to have the Room create enchanted spell casters that would hit him with legitimately harmful spells, like the severing charm, Diffindo.
The room shifted at his request, forming an octagonal space. Enchanted casters appeared at random points along the walls.
Julian stood at the center and transfigured his clothing down to a single pair of thick shorts, reinforced with metal at the groin to protect that specific area from any spell that might target it.
"Begin," he said calmly.
...
A small red spell shot at him from the left.
"PROTEGO!" he snapped, fast and sharp.
All he got was a blue spark from his wand.
His eyes narrowed a fraction before the spell struck his left arm, tearing open a nasty gash.
Julian grit his teeth, ignored the pain, and locked his focus back onto the room for the next attack.
This time it came from behind. He spun and leveled his wand at a beam of purple light.
"PROTEGO!"
Again, only a spark.
And the impact left him with a bruise for his effort.
...
Julian quickly lost count of how many times he was hit by painful spells. When the room finally warned him that dinnertime was approaching, he stopped the training in an awful condition.
He pulled a wiggenweld potion from Greed, popped the glass top, and forced down the bitter liquid. Cuts, bruises, and burns scattered across his body began to close, scab, and settle as the potion did its work, and he felt relief almost immediately.
"NEN!" he cast.
Water streamed from his wand and washed away the dried and fresh blood, sweat, and tears clinging to his skin.
...
The last few hours had been little more than suffering. He'd barely made any progress at all. The most he'd managed was a flickering blue light at the wand's tip, and nothing more.
In exchange, he'd been cut, struck, and burned across multiple parts of his body.
Julian knew exactly why he'd failed. Part of it was that the spell was far beyond his current casting level.
The other part was worse.
His own nature.
He didn't enjoy being hurt, but he didn't fear it either, and it was that exact trait, normally considered admirable, that was preventing him from making real progress.
...
He released the transfiguration on his robes, covering the many healing injuries beneath the fabric. Then he steadied his expression and left the room, heading toward the Great Hall while keeping any hint of pain off his face when he moved.
The terror sparked by that morning's news hadn't faded at all, and Julian didn't think it ever would as long as Grindelwald remained free.
...
Julian sat beside Harry and across from Ron at the Gryffindor table, waiting for the food to arrive.
When it did, he rolled up his sleeves slightly so they wouldn't brush his meal.
Ron and Harry both reacted at once.
"Bloody hell!" Ron blurted, staring. "What happened to your arms?!"
Julian's forearms were covered in scabs and discolored patches of healing skin.
"Don't worry, I'm fine," Julian said, keeping his tone even. "I did it to myself, honestly. I overestimated myself a bit, and this is the result."
...
"What were you trying to do that could cause that?" Hermione asked from the other side of Harry, her expression tight with concern.
"Just trying to learn the shield charm," Julian said with a shrug. "Turns out not being afraid of injury can actually be detrimental to spellcasting."
Hermione blinked. "Isn't that a fourth-year spell?"
Julian nodded. "Yes. But with a madman willing to sacrifice an entire town running around, I figure this," he gestured at his injuries, "is a small price for a little safety."
The table went quiet, because he wasn't wrong.