I see that a lot of people are having a problem with the last chapter so I want to resolve it while I still can.
First of all - it's not nerf, he didn't get any weaker and if you are aware, people don't work the way that they grow stronger at the same speed. The start is always faster and then it only drops down. At least that's how I think it works because when I started to go to the gym. When I started, I could squat only 50 kg and after 4 months of training I could squat 100 kg, however, after another 4 months I improved only to 105 kg. Do you understand what I am trying to say here? (That's also one of the reasons why I stopped going there, slow progress but that's the limitation of the human body and MC is still human too...) ¯\_(ツ)_/¯
Of course, he has the system, however, you must realize that each higher level of class requires more experience and is also slower to level up if you are killing weaker opponents than you. MC has a problem finding really strong opponents, he can almost instakill everything with his flames because it is either too slow to dodge and if it is an agile beast then it has bad defenses which means he can finish it off with his sword. You don't seriously expect opponents that you can instakill to give you large amount of experience.
This system doesn't take (MC's level)V(imaginary level of your opponent), it takes (MC's stats)V(Strength of the opponent) (Ofc the level of the current class also matters, for example, the level 1 class would get 100 XP for killing that turtle opponent Mc killed previously but level 9 class would get much, much less. He has to kill the strong opponents but they are hard to find.
Previously he was leveling fast because his stats were low and every opponent was around his level or slightly weaker.
To be honest, right now it is better for him to hunt Teigu users than Danger beasts because it is extremely hard to find beasts that are strong enough to rival Teigu weapons and also have some kind of element in them.
NO NERF - (you don't understand the term of nerfing, he didn't get any weaker and his growth wasn't even limited, just him getting that warrior on 10th level gave him all its stats passively. That isn't a big buff when he can level up another class while having stats from the old one?) MC didn't get any weaker and everyone who was already strong like Esdeath, for example, is also growing much slower. Also, there is a big difference in STR: 5 -> 6 upgrade and STR: 55 -> 56. It is the same 1 point but on higher stats, it gives many more benefits than on low numbers. Not to mention that he still has his free stat points.
I am also planning to give him another class in the next few chapters that will give him more passive benefits instead of many stats.
If you are still unconvinced I can also rewrite it but then again the MC would be ridiculously strong. Right now, only Esdeath can entertain him while he would be using his full power (without the [Absolute Dominance] and [Insatiable Hunger] just to remind you his Absolute Dominance gives him +60%!!! to all his stats and increases the density and temperature of his flames.).
If you are also concerned about the challenge then MC is still human so when many Teigu users group up at him, even he would be forced to use those skills. Fighting against numerous strong opponents is always tricky, not to mention if they have good teamwork.
Not sure if it's everything but I am sure that I wrote also some bs but I am too lazy to re-read it.
Ofc I won't say that everything is logical, I wanted to limit it a little bit so he could also find some challenge and also level up his other classes. Like I mentioned before, right now, it is more beneficial for him to hunt strong humans instead of beasts. Humans are intelligent, cunning and also very strong. It's harder to fight against a human than a beast unless that beast is also intelligent.