"Humans are not designed to take such a toll as this," he said, struggling to meet Sam's eyes. He wanted to think of something different - something easier and friendlier than one of his boys in pain, and not being able to do much to stop it. The more he tried to focus on Sam, the more the thoughts squeezed out of his head, like wringing water from a towel. But this was important. This was Sam.
He didn't really need air, but he took a deep breath in anyways. "Alright. I will try, then. Lie still, Sam. This... may hurt somewhat." Healing usually brought relief, not pain, but... this was no ordinary situation.
At one point, with stolen power coursing through his being, he would have been able to heal Sam easier than blinking. In comparison to when he'd still had the souls of Purgatory to siphon from, Castiel's grace was barely a spark now. Of course, as the arrogant would-be God, he had lacked the compassion to do something like this, so there was that in his favor currently.
So he reached out his hand once again to brush against Sam's brow, reached out with that tiny spark, and started trying to knit the broken parts of Sam's body back together. He tried to concentrate on Sam's major organ systems - heart and lungs and liver and kidneys. The spellwork from the Trials was resistant, even belligerent. He couldn't wipe out the source, so instead, he concentrated on the spread of the damage, working from the outside in. At the very least, this could sooth some of the worst of the pain and ease the strain on his body. When they were finished here, he could send Sam to sleep, to allow his body's own healing to carry it further. But for now, he concentrated.