I think the only reason Shun was able to walk as far as he did with Sayaka before collapsing, was due to adrenaline.

I’ve neve been on an adrenaline rush before(that I know of), but I have heard from people that it is some powerful stuff. Like, ‘you can’t even feel that broken arm’ powerful.

So, Shun sees a comrade in danger and gets a bust of adrenaline as he rushes to save them-this gives him the burst of speed and strength as well as the temporary immunity to pain to keep him on his feet. We don’t see what happened in the debris cloud, but I would be willing to bet he used his body as a shield and took some heavy damage before getting out.

He lasted just long enough to walk out, set her down once he knew she was safe, then collapsed as everything metaphorically crashed around him(I don’t know how long an adrenaline rush can last, apparently it varies from person to person and situation to situation). Especially if he actually took some serious damage. He may not feel the pain, but the injuries themselves may be enough  to render him out cold.

Besides, suspension of disbelief people. You’re watching an anime with lethal holograms, multiple dimensions, and card games used to kill people.