WiMaiRi
Body, pain tolerance, soul,........... If he wants to improve these things, he better hope his next world he goes to is Douluo Dalu (Soul Land). In that Soul land, he can get bloodlines like dragon bloodlines that improve the body and make him have monster strength/beast in human form. His pain tolerance will shoot through the roof and he will get added lifespan. His soul will get stronger, he would get a martial spirit of his own that he can also strengthen. Douluo Dalu is the answer to all his problems, he can also use its resources to strengthen his allies. And Nagini could benefit from it the most, after all, in that world, beasts can take human form when they reach a certain age and strength. So him, Perona, Nagini and the rest of his companions trapped in the apocalypse world (for now) would benefit the most from it. Other worlds he can go to where he can increase his body, pain tolerance and soul, Naruto world. Seriously, Douluo Dalu,........... consider it. Plus lots of young masters to smack around. If you don't make it his next world or don't plan on sending him there anytime soon, atleast make him go there sometime in the future, or if you don't wanna write it, then mention it briefly in the story like saying, the MC went there and copied all kinds of bloodlines and martial spirits from there being multiple tool, body and beast martial spirits and it helped alot. Tauriel could still get her phoenix power that way, copying the phoenix bloodline or martial spirit to her. Then you continue with the story.