He exchanged his soul to make his previous life stop, becoming a demon in the process.
As of right now, he knows little of how to make materials. I was planning on scaling this off of his mental deduction, and make him only be able to make simple things such as condensed carbon for an extra bit of defense. He would only be able to make more complex things if he had a source of energy nearby he could bond to when exposed, such as electricity. Any special things of particular importance such as a material made on site, he'd have to do that in thread, and I would hold off on anything like that until he got more used to his body and a bit smarter @Mental Deduction, potentially specifically going through memorization training in threads multiple times. This would drain his stamina, though due to his body he'd have a high reserve, but he'd still have limits, also planned on scaling this off of his physical stats and things like willpower/determination and focus.
How often he can shift matter? I'd also scale this somewhat off of his physical stats. I'd say he can shift pretty fast, for materials that are straightforward such as carbon i'd say he could convert in a matter of a few seconds currently. The more complex a material is, the longer it'd take naturally, though with increased speed the more power sources he has available.
Due to his body being largely ambiguous and transformative, it would be able to adapt to certain things that it's introduced to, and adaptation would depend on how volatile the invading material would be. Essentially, the more he's exposed to something within a short period of time, the more resistant his body becomes to it. How quickly would also reasonably depend on how powerful the invasion was, such as: the time taken to mitigate the damage from a fire based ability of a weaker opponent and a stronger opponent being potentially vastly different. This would also be somewhat scaled off of physical attributes and things such as willpower/determination and focus.
Bonding is meant to be mainly used for energy. When exposed to a usable power source, he would begin molecularly bonding with it and burning it as fuel for his abilities. This can also be used in the instance a toxin that is able to effect him or an acid is introduced to his body, through enough exposure to it, his body could begin fusing with it, gaining it's properties, mitigating damage from it, and perhaps learning how to recreate it, even if the substance is unique. This follows the same logic of adaptation: the stronger the substance is, the longer it would naturally take.
KILL IT WITH FIRE!
Yes, joke aside, he can be damaged, but he's hard to physically take down. While blunt attacks wouldn't do much from the lack of particular weak points, slashing him repetitively could take him down a notch from the damage stacking up. When he changes his body, it takes energy, and when no extra power source is available, this burns his stamina, and he can run out of that, though this method could still take a long time and not be overly viable. A better option would be something such as previously mentioned, fire. Chloranthy can burn, and even if eventually he can start mitigating the damage, enough can still hurt him. Fire also cannot be really absorbed as fuel, so he would be much less able to adapt to fire than something such as electricity. With the same reasoning, strong acids would also be effective since they would destroy his matter and not just split it apart. Freezing him could also be somewhat effective as the cold, while probably a bit easier to adapt to, could still kill off molecules. Chloranthy has no weak points, so one just has to think of what could hurt his body as a whole effectively. With that logic, physical attacks could be made viable if some sort of area of effect were to be added, such as vibrations, as those would damage individual molecules much more easily, and the chaos they cause would also make it harder to change his body.
As all of his abilities are physically based and physically draining, the vast majority of them are based off of his physical statistics. Energy around could also potentially buffer these abilities and mitigate the physical drain. However, Chloranthy can still very much be hurt, as he is entirely physical, and many different things can hurt physical matter, one just has to think.
Hope this fixes things.