@B1L0
The manga skips over a lot of his training I think, so it does seem like it's been a shorter timeframe? If that's what you're trying to get at, correct me if I'm wrong.
I'll have to reread the wn to clarify, I don't recall him kicking a shell.
Yes it doesn't prevent physical harm, which is why
1. He's been undergoing some pretty harsh training to improve his resilience, to the point where Alpha has judged him ready for an exoskeleton. Can't really argue with an omniscient presence.
2. He's also using some pretty souped-up drugs to improve recovery rate, as in literally "regenerating" his injuries. IIRC, he could feel himself being stitched up while the stress tore his body apart. Wonders of sci-fi nanobots I guess.